LL 


CHEMISTRY  FOR 
PHOTOGRAPHERS 


" 


BOOKS  ON  PHOTOGRAPHY 


PRACTICAL 
PHOTOGRAPHY    SERIES 

Edited  by 

FRANK  R.  FRAPRIE,  S.M.,  F.R.P.S. 
Editor  of  American  Photography 

1.  THE  SECRET  OF  EXPOSURE 

2.  BEGINNERS'  TROUBLES 

3.  How  TO  CHOOSE  AND  USE  A  LENS 

4.  How  TO  MAKE  PRINTS  IN  COLORS 

5.  How  TO  MAKE  ENLARGEMENTS 

6.  How  TO  MAKE  PORTRAITS 
Cloth,  50  cents  net;  Paper,  25  cents  net 

CHEMISTRY  FOR  PHOTOGRAPHERS 

By  WILLIAM  RUTHVEN  FLINT,  PH.D. 

Cloth,  $2.00  net 

AMERICAN  PHOTOGRAPHY  EXPOSURE 

TABLES.      Cloth,  25  cents  net 

AMERICAN  PHOTOGRAPHY,  a  monthly 

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THE  PAGE  COMPANY 
BOSTON  -  -  MASS. 


•CHEMISTRY 

FOR 

PHOTOGRAPHERS 


By 
WILLIAM  RUTHVEN  FLINT,  PH.  D. 


THE  PAGE  COMPANY 
BOSTON          -          -          PUBLISHERS 


Copyright,  1916,  by 
American  Photographic  Publishing  Co. 


BEVERLY  PRINTING  COMPANY 
BEVERLY,  MASS. 


TO  MY  WIFE 

As  a  Souvenir  of  the  Many  Photographic  Excursions 
We  Have  Enjoyed  Together 

THIS  BOOK 

IS 
AFFECTIONATELY  DEDICATED 


371606 


PREFACE 

THE  purpose  of  the  following  pages  is  two-fold.  First, 
the  chemical  principles  whose  application  forms  the 
foundation  of  the  photographic  art  are  set  forth  in  a 
manner  which  it  is  thought  will  prove  both  intelligible 
and  interesting.  In  order  to  fix  these  principles  in  the 
reader's  mind  and  at  the  same  time  to  aid  him  in  the 
acquisition  of  a  better  chemical  technique,  the  subject 
matter  has  been  so  arranged  as  to  permit  the  intro- 
duction of  a  series  of  illustrative  experiments. 

Second,  without  in  any  way  interfering  with  the  fore- 
going intent,  it  has  been  possible  to  add  very  materially 
to  the  practical  value  of  the  book  by  incorporating 
much  useful  chemical  and  photographic  information  in 
the  way  of  solubilities,  formulas,  etc.  This  information 
has  been  so  simplified  and  tabulated  as  to  make  it 
exceptionally  convenient  for  reference  purposes. 

To  make  special  acknowledgment  of  each  of  the 
authorities,  chemical  and  photographic,  which  must  be 
consulted  in  the  preparation  of  such  a  volume  as  the 
present  one  would  be  both  cumbersome  and  superfluous. 
The  literature  of  the  subject,  technical  as  well  as  popu- 
lar, has  been  freely  utilized,  and  the  author  would  here 
express  his  indebtedness  to  all  the  writers  and  investi- 
gators whose  work  has  been  of  so  much  assistance,  but 
whose  names  it  is  impossible  to  enumerate. 

The  author's  thanks  are  particularly  due  to  Mr. 
Lorenzo  Whiting  of  Pasadena,  California,  who  has  care- 

IX 


fully  performed  and  checked  most  of  the  formal  ex- 
periments included  in  the  text. 

In  committing  this  little  book  to  the  reader's  hands 
the  author  sincerely  hopes  that  it  will  be  found  helpful, 
not  alone  in  the  saving  of  time  and  materials  which 
must  come  from  a  better  understanding  of  the  proc- 
esses of  photography,  but  also  in  an  augmenting  of  the 
amateur's  interest  and  pleasure  in  a  delightful  avoca- 
tion. 


CONTENTS 


PREFACE    . 
CHAPTER  I. 

Introductory    

Page 
IX 

I 

^  CHAPTER  II. 

Chemical  Reaction 

Q 

CHAPTER  III. 

Light  and  Chemical  Reaction  . 

27 

CHAPTER  IV. 

Applied    Photo  -  Chemistry  of 
Silver  Salts        .... 

'  43 

CHAPTER  V. 

Chemistry  of  Development 

61 

CHAPTER  VI. 

Chemistry  of  the  Fixing  Pro- 
cess        

7Q 

CHAPTER  VII. 

After-treatment  of  the  Nega- 
tive        

/     / 

Q3 

CHAPTER  VIII. 

Printing  Processes  with  Silver 
Salts      .      .      ...      . 

vx 

1  08 

CHAPTER  IX. 

Printing   Processes   with    Iron 
Salts      ......      . 

no 

CHAPTER  X. 

Printing  Processes  with  Chro- 
mium Salts       v     ...      .     % 

s 

153 

CHAPTER  XI. 

The  Chemicals  of  Photography 

169 

APPENDIX       ^ 

.         .       '  *         •_,"-•• 

183 

INDEX 

200 

Chemistry 

for  Photographers 


CHAPTER  I 
INTRODUCTORY 


THE  two  most  marked  tendencies  in  the  photo- 
graphic art  of  today  are  represented  by  two 
different  types  of  workers.  One  of  the  types  includes 
those  who  are  the  most  advanced  workers  of  the  day, 
the  pictorialists,  whose  whole  endeavors  are  being 
earnestly  directed  toward  the  expression  of  ideas 
through  the  medium  of  photographic  processes  suitably 
controlled.  The  other  type  comprises  the  scientific 
workers,  interested  primarily  in  the  investigation  of 
the  chemical,  physical,  and  physico-chemical  principles 
upon  which  the  science,  if  we  may  call  it  such,  of 
photography  rests.  Both  of  these  classes  have  already 
made  their  indelible  impress  upon  the  art,  the  former 
largely  in  the  extreme  evolution  of  the  colloid-dichro- 
mate  methods  by  which  the  pictorialists  are  able 
with  the  greatest  freedom  to  give  play  to  their  artistic 
feelings  in  their  prints.  The  influence  of  the  latter 
has  been  felt,  not  only  in  the  improvement  of  manu- 
facturing processes  and  products,  but  in  the  revision 
of  our  ideas  upon  the  subject  of  development  as  a  whole, 


^       CHEMISTRY  FOR  PHOTOGRAPHERS 

and  in  the  attempt  at  standardization  of  many  methods. 

These  two  types  appear  to  be  at  the  extremes  of 
photography,  the  pictorialists,  on  the  one  hand,  sub- 
ordinating the  scientific  and  the  mechanical  as  much  as 
possible  and  laying  stress  upon  the  purely  artistic,  and 
the  scientific  workers  giving  practically  the  whole  of 
their  attention  to  improving  the  quality  and  increasing 
the  accuracy  of  delineation  of  the  photographic  records 
in  which  they  are  interested.  Between  these  two  rather 
widely  separated  groups  are  the  great  body  of  amateurs 
and  a  large  proportion  of  professionals,  most  of  whose 
photographic  operations  are  still  carried  on  by  rule-of- 
thumb  methods.  Nevertheless,  the  numbers  of  en- 
thusiastic amateurs  who  go  snapshotting  more  or  less 
at  random  about  the  country  testify  amply  to  the 
fascinating  power  of  photography  even  when  its 
processes  are  empirically  worked.  Doubtless  it  is  from 
watching  the  wonderful  succession  of  changes  through 
which  their  picture  goes  as  it  passes  from  one  solution  to 
another,  together  with  the  certainty,  greater  or  less 
according  to  their  skill,  of  being  able  to  produce  in  the 
event  a  particularly  desired  result,  that  for  most  ama- 
teurs a  great  part  of  the  pleasure  in  their  craft  comes. 
It  is  with  the  purpose  of  helping  to  increase  and  extend 
this  fascination  and  pleasure,  by  adding  a  clearer 
understanding  of  what  goes  on  in  these  processes,  that 
we  shall  proceed,  in  the  succeeding  chapters,  to  discuss 
the  chemical  principles  which  underlie  them. 

The  endeavor  will  be  made,  with  what  success  the 
reader  must  judge  for  himself,  to  describe  and  explain 
in  an  elementary  way  the  manner  in  which  chemical 
reactions,  delicate  and,  very  many  of  them,  exceedingly 
intricate,  are  so  controlled  as  to  yield  both  the  photo- 


INTRODUCTORY  3 

graphs  of  the  everyday  amateur  and  the  works  of  art 
of  the  skilled  pictorialist.  The  author  has  been  led  to 
make  this  attempt  because  of  the  conviction,  impress- 
ing itself  more  and  more  strongly  upon  his  mind  with 
the  growth  of  his  own  experience  and  the  extension  of 
his  reading  and  study  in  photographic  matters,  that 
the  admittedly  low  standards  of  most  amateur  and  a 
large  proportion  of  professional  work  are  due  mainly 
to  two  causes.  The  first  of  these  is  the  failure  to  under- 
stand the  chemistry  of  photographic  processes,  and 
the  second,  the  lack  of  appreciation  of  the  principles  of 
composition,  perspective,  and  those  qualities  which 
go  to  make  a  picture.  Now,  of  course,  not  every  photo- 
graph needs  to  be  a  "picture,"  and  no  one  can  right- 
fully quarrel  with  the  amateur  who  prefers  to  make 
photographically  a  purely  literal  record  of  his  ex- 
periences, whether  everyday  or  vacation.  However 
much  the  advanced  pictorialist  may  pity  such  an 
amateur,  there  is  a  great  deal  of  interest  and  charm  in 
the  making  of  straight  record  photographs.  But  the 
photographers  who  make  them  ought,  for  the  sake  of 
their  friends  if  not  for  their  own,  to  try  to  make  them 
as  perfect  as  possible.  This  is  the  reason  why,  of  the 
two  causes  just  mentioned,  the  chemical  was  placed 
before  the  artistic;  for  it  must  be  from  the  teeming 
ranks  of  the  aspiring  amateur  "straight"  photographers 
that  the  majority  of  "advanced"  and  "pictorial" 
workers  are  recruited.  The  first  essential,  therefore, 
is  to  inculcate  as  widely  as  possible  a  more  intelli- 
gent conception  of  the  chemical  foundations  upon 
which  photography  rests,  in  order  that  the  average 
photographer  may  better  understand  his  medium, 
improve  his  practice,  and  thereby  perfect  his  results. 


4       CHEMISTRY  FOR  PHOTOGRAPHERS 

There  has  in  the  past  been  prevalent  among  more 
or  less  advanced  workers  the  fear  that  their  art  might 
become  too  mechanical,  and  for  this  reason  there  seems 
to  have  been  almost  a  prejudice  against  putting  the 
formulas  for  developing  and  other  solutions  into  reason- 
able chemical  shape.  Such  photographers,  for  example, 
in  recommending  a  recipe  for  developer,  would  suggest 
adding  "a  pinch  of  pyro"  to  a  specified  mixture  of 
alkali  and  sulphite,  or  "as  much  pyro  as  will  go  on  the 
point  of  a  penknife."  At  the  present  time  there  does 
indeed  appear  to  be  a  deal  of  the  mechanical  and 
automatic  in  photography.  It  has  come  about,  how- 
ever, not  from  the  standardizing  of  formulas  and 
manipulations,  but  from  the  output  of  numerous,  and 
it  must  be  admitted  extremely  convenient,  prepared 
powders  and  solutions,  most  of  which  need  the  addition 
of  nothing  but  water.  The  user,  therefore,  is  generally 
ignorant  not  alone  of  the  concentrations  of  the  active 
agents  in  his  solutions,  but  often  of  the  very  identity 
of  their  constituents.  He  performs  certain  manipula- 
tions according  to  the  printed  directions,  but  naturally 
can  comprehend  little  of  their  real  import.  The  author 
hopes  in  the  following  chapters  to  aid  in  changing  this 
state  of  things,  by  helping  the  worker  to  understand, 
to  take  an  example,  that  when  an  exposed  plate  is 
developed  there  is  an  equivalence  between  the  quantity 
of  silver  reduced  and  the  amount  of  developing  agent 
oxidized;  that  it  is  this  reaction  which  uses  up  the 
developing  agent,  aside  from  its  spontaneous  oxidation 
by  the  air;  and  that  because  by  these  oxidations  the 
concentration  of  the  reducing  agent  is  diminished,  it  is 
better,  for  the  sake  of  clean,  uniform  negatives,  to  use 
a  given  small  portion  of  the  developer  solution  but 


INTRODUCTORY  5 

once,  and  then  to  throw  it  away,  rather  than  to  use  it 
over  and  over  again  until  it  is  exhausted.  The  quantities 
of  substance  of  which  photographic  images  are  com- 
posed are  so  minute  that  the  concentrations  of  the 
active  agents  with  which  these  images  are  treated  have 
to  be  controlled  with  considerable  care.  It  is  one  of  the 
axioms  of  physical  science  that  the  same  causes  always 
operate  to  produce  the  same  effects.  Since  photog- 
raphy depends  so  largely  upon  the  operation  of  physical 
and  chemical  laws,  it  ought  to  be  plain  that  obedience 
to  them  is  the  key  to  success,  represented  by  well- 
executed,  permanent,  and  attractive  prints. 

As  all  physical  science  rests  upon  an  experimental 
basis,  and  especially  since  a  much  better  idea  of  the 
laws  and  phenomena  of  a  science  can  be  had  by  actually 
performing  for  oneself  even  the  simplest  of  experiments 
than  by  any  amount  of  reading  about  them,  the  author 
has  incorporated  with  the  text  of  this  discussion  a  set  of 
selected  experiments  intended  to  illustrate  the  principles 
which  are  developed.  He  emphatically  recommends 
that  the  reader  perform  every  one  of  these  experiments 
in  course.  No  elaborate  apparatus  is  needed  for  them, 
and  the  chemicals  are  those  which  are  most  commonly 
used  by  photographers.  Neither  is  any  special  equip- 
ment necessary,  but  either  the  kitchen  sink  or  the 
amateur's  ordinary  darkroom  will  serve  quite  as  well  as 
though  it  were  the  most  fully  equipped  chemical 
laboratory.  In  working  them  out  it  will  be  found  help- 
ful to  determine  first  of  all  exactly  what  is  intended  to 
be  illustrated  in  each  case,  that  is,  the  object  of  the  ex- 
periment. Then  pay  close  attention  to  everything  that 
goes  on  as  the  reaction  progresses.  There  is  nothing 
too  insignificant  to  be  noted  down.  A  class  of  young 


6       CHEMISTRY  FOR  PHOTOGRAPHERS 

chemistry  students  once  discovered  that  in  the  very 
simple  operation  of  boiling  a- little  water  in  a  test  tube 
there  are  some  twenty-five  different  phenomena  which 
can  be  observed.  It  is  in  this  way  that  one's  powers  of 
observation  are  cultivated,  and  that  is  an  end  that  is 
very  well  worth  while  in  itself.  And,  finally,  all  the  facts 
which  are  observed  ought  to  be  written  down,  with  such 
inferences  as  can  be  drawn  from  them.  By  this  means 
they  will  be  the  better  fixed  in  mind  and  can  easily  be 
referred  to  later,  if  necessary,  without  the  experiment 
having  to  be  repeated.  The  habitual  use  of  a  notebook 
in  which  are  recorded  facts,  observations,  comments, 
etc.,  is  a  good  custom  for  anyone  to  adopt,  and  it  is 
particularly  so  for  the  photographer,  who  ought  to 
make  a  practice  of  recording  the  data  of  all  his  ex- 
posures. While  the  author  feels  that  his  work  will  be  of 
the  most  help  if  his  readers  will  conduct  the  experiments 
as  they  are  described  and  along  the  lines  just  suggested, 
nevertheless  he  has  so  arranged  the  directions  for  doing 
them  that  no  one  need  be  deterred  from  studying  the 
book  by  inability  to  perform  some  or  all  of  them. 

So  much  has  been  written  upon  the  darkroom,  per  se, 
and  upon  its  plan  and  arrangement  that  we  need  spend 
no  time  in  discussing  it  here.  Besides,  almost  every 
photographer  has  his  own  special  ideas  about  it,  or  has 
his  own  special  conditions  to  contend  with  in  its  location 
and  construction.  It  will  be  sufficient  and  quite  satis- 
factory to  assume  that  he  has  devised  an  arrangement 
which  suits  his  convenience  as  well  as  possible.  And 
beyond  this  we  need  not  go,  since  we  have  already  re- 
marked that  the  place  in  which  the  experimental  work 
is  done  is  of  little  consequence  beside  the  manner  in 
which  it  is  done.  But  there  are  certain  simple  items  of 


INTRODUCTORY  7 

apparatus  which,  while  often  found  in  the  photog- 
rapher's workroom,  we  had  best  enumerate  for  the  sake 
of  any  one  who  may  not  have  them  at  hand.  The  first, 
and  indispensable,  is  a  balance  and  set  of  weights.  The 
balance  may  well  be  the  regular  photographic  scales,  or 
either  the  very  cheap  hand  scales  with  horn  pans  or  the 
slightly  more  expensive  "coarse  weighing"  balance 
supplied  by  the  dealers  in  laboratory  apparatus.  It 
need  not  cost  more  than  $3.00  or  $3.50  for  the  one  last 
mentioned,  which  must  be  capable  of  weighing  to  0.05 
gram  at  least  and  up  to  100  grams.  With  the  photo- 
graphic scales  there  usually  is  furnished  a  set  of  grain 
weights,  but  they  will  not  be  useful  in  connection  with 
the  present  work.  Curiously  enough,  these  sets  of 
weights  are  commonly  a  mixture  of  avoirdupois  and 
apothecary  weights.  This  ought  to  be  sufficient  com- 
ment upon  the  system.  The  set  required  runs  from  a 
5o-gram  piece  down  to  0.05  gram  and  costs  not  more 
than  a  dollar.  There  is  much  satisfaction  in  the  use  of  a 
good,  substantial  balance  which  does  not  easily  get  out 
of  order,  and  it  is  excellent  advice  to  get  the  best  that 
can  be  afforded.  But  there  is  no  need  to  exceed  the 
specifications  given  for  capacity  and  sensitiveness.  In 
lieu  of  means  for  weighing  out  the  chemicals  for  the 
experiments  himself,  the  worker  who  has  no  balance 
can  probably  persuade  some  druggist  friend  to  weigh 
them  out  for  him.  Besides  the  ordinary  darkroom 
equipment  of  trays,  tanks  for  developing  and  fixing, 
and  glass-stoppered  bottles  for  solutions,  etc.,  it  will 
be  most  convenient  to  have  a  few  items  of  glassware. 
These  are  obtainable  from  any  dealer  in  laboratory 
apparatus  and  for  a  very  moderate  outlay.  The  list  of 
articles  will  be  found  in  the  appendix  at  the  back  of  the 


8       CHEMISTRY  FOR  PHOTOGRAPHERS 

book,  where  is  also  included  a  list  of  the  chemicals 
needed  in  the  experiments. 

The  subject  of  photography,  as  we  have  intimated, 
is  a  very  wide  one.  Viewed  from  whatever  direction, 
in  its  enormously  varied  applications,  in  its  purely 
scientific  investigations,  and  in  its  adaptation  to  the 
uses  of  the  amateur,  its  development  has  been  little 
short  of  marvelous.  What  further  progress  is  still  to  be 
made  only  the  future  can  tell.  And  yet,  from  the  photo- 
micrographs of  the  biologist  to  the  stellar  photographs 
and  the  spectroheliograms  of  the  astronomer,  from  the 
record  photography  of  the  amateur  to  the  best  work  of 
the  great  pictorialists,  the  whole  structure  of  the  art  is 
based  on  the  one  simple  fact  of  the  remarkable  action 
of  light  upon  certain  chemical  compounds.  The  succeed- 
ing chapters  will  treat  of  this  action  and  of  these  com- 
pounds in  a  way  that  the  author  hopes  may  be  found 
not  only  intelligible  and  interesting  but  instructive  and 
helpful. 


CHAPTER  II 
CHEMICAL  REACTION 


ONE  of  the  most  general  observations  which  can 
be  made  about  the  world  in  which  we  live  is  that 
nothing  remains  unchanged  for  any  considerable  period 
of  time.  The  appearance  of  a  landscape  under  bright 
sunshine  is  continually  subject  to  change  as  the  shadows 
shift  with  the  altering  position  of  the  sun.  The  study 
of  these  variations  in  the  same  view  forms  a  most  useful 
exercise  for  the  photographer  who  is  interested  in 
landscape  work.  Beneath  a  clouded  sky  the  view  pre- 
sents a  totally  different  aspect  from  any  of  these 
appearances.  If  we  pass  to  a  consideration  of  the  sky 
oti  such  a  cloudy,  gray  day,  we  shall  see  that  even  the 
clouds  are  not  uniform.  There  are  lighter  and  darker 
grays.  By  fixing  the  attention  upon  a  single  patch  of 
gray  cloud,  we  find  that  the  quality  of  grayness  is 
changing  in  both  tone  and  texture. 

Let  us  walk  down  beside  the  clear  pool  which,  with 
its  elliptical  outline  and  its  reflections,  forms  such  an 
attractive  feature  in  our  imaginary  landscape.  Whether 
the  air  is  still  or  whether  a  breeze  ripples  the  water; 
whether  we  see  in  its  mirror-like  surface  the  clear  blue 
of  the  sky  or  the  rolling  masses  of  a  dazzling  white 
thunderhead,  surely,  we  may  think,  here  is  a  body  that 
is  not  changing.  Water  is  water ;  the  observed  changes 
are  external  to  the  body  itself  and  are  only  appearances. 
The  physicist  comes  along  with  a  little  delicate  ap- 
paratus and  shows  us  that  the  vapor  of  water  is  con- 
tinually rising  from  the  surface  of  the  pool.  And  more 


io  CHEMISTRY  FOR  PHOTOGRAPHERS 

than  this,  by  certain  experimental  and  mathematical 
considerations  he  can  also  prove  to  us  that  water  vapor 
is  at  the  same  time  condensing  from  the  air  into  the 
pool.  Here  then  it  seems  is  a  different  sort  of  change 
from  the  changes  in  the  appearance  of  things  of  which 
we  have  been  speaking.  And  yet  it  is  only  seemingly 
different  because  invisible  to  the  eye.  The  physicist  will 
tell  us  that  all  these  changes  are  physical  changes 
because  the  identity  of  the  bodies  concerned  has  not 
in  any  case  been  really  affected.  For  example,  whether 
water  be  liquid  water  or  vapor  of  water,  its  identity  is 
the  same ;  only  its  condition  has  suffered  a  change. 

Let  us  now  go  around  the  pool  to  the  big  tree  on  the 
other  side.  We  know  that  its  beautiful  green  leaves, 
which  came  out  of  the  winter  buds  early  in  spring,  will 
be  changed  into  an  autumn  glory  in  the  coming  fall. 
But  for  the  time  being  they  are  just  green  leaves,  and 
cannot  it  be  said  that  they  are  unchanged,  temporarily 
at  least?  The  answer  now  comes  from  the  chemist.  By 
the  very  simple  device  of  putting  a  leaf  into  a  tube  full 
of  water  and  standing  it  in  sunlight,  he  can  show  that 
all  the  while  the  sun  is  shining  upon  it  the  leaf  is  busily 
engaged  in  consuming  one  of  the  constituents  of  the 
air,  namely,  carbon  dioxide,  retaining  within  itself  the 
carbon  with  which  to  feed  the  rest  of  the  plant,  and 
breathing  out  the  oxygen.  That  the  bubble  of  gas 
collected  at  the  top  of  the  tube  really  is  oxygen  can  be 
proved  by  bringing  into  it  the  end  of  a  glowing  splinter 
of  wood,  whereupon  the  spark  will  burn  much  more 
brightly  than  in  the  air. 

So  from  the  chemist  we  learn  that  in  this  operation 
of  the  green  leaf  under  the  influence  of  light  there  are 
three  bodies  concerned.  First,  there  is  carbon  dioxide, 


CHEMICAL  REACTION  1 1 

a  colorless  gas  in  which  a  glowing  splinter  is  extin- 
guished; second,  oxygen,  also  a  colorless  gas,  but  a 
supporter  of  combustion;  and  third,  the  carbon. 
Dismissing  the  leaf  from  further  consideration  with  the 
remark  that  it  is  able  to  assimilate  this  carbon  in  such 
a  way  as  to  store  it  up  in  the  woody  structure  of  the 
plant,  we  may  note  that  carbon  is  ordinarily  a  black 
solid  substance,  quite  different  in  every  way  from 
either  of  the  other  two.  Stating  the  proposition  again 
and  in  another  form,  we  may  say  that  it  is  possible  to 
take  the  colorless  gas,  carbon  dioxide,  and  to  get  from 
it  another  colorless  gas,  oxygen,  and  black  solid  carbon. 
This  would  be  plainly  a  different  kind  of  change  from 
the  vaporization  of  liquid  water  into  water  vapor  or  the 
condensation  of  water  vapor  into  liquid  water,  which,  as 
we  have  seen,  are  physical  changes  because  the  identity 
of  the  water  is  unaffected.  It  is,  indeed,  a  chemical 
change,  for  the  identity  of  the  carbon  dioxide  is  lost, 
and  two  new  bodies  whose  identity  is  different  are 
obtained.  Thus  we  may  define  as  chemical  changes  all 
those  changes  by  which  the  identity  of  the  bodies  con- 
cerned is  altered. 

By  much  patient  investigation  chemists  have  proved 
that  all  the  many  thousands  of  different  kinds  of  bodies 
are  various  combinations  made  up  from  only  about 
eighty  really  different  kinds  of  matter.  These  eighty 
substances  are  known  as  the  chemical  elements,  be- 
cause none  of  them  has  so  far  ever  been  separated  into 
anything  simpler.  When  any  two  or  more  of  the 
elements  combine  in  such  a  manner  as  to  form  a  definite 
substance,  a  chemical  change  occurs  because  there  is  a 
change  of  identity.  This  change  is  called  a  chemical 
reaction,  or  simply  a  reaction.  Thus  the  chemical 


ia   CHEMISTRY  FOR  PHOTOGRAPHERS 

change  already  considered,  that  of  the  carbon  dioxide 
into  carbon  and  oxygen,  is  a  reaction.  It  is  possible  to 
state  reactions  very  simply  in  terms  of  the  substances 
involved  and  in  the  form  of  equations,  and  for  the  sake 
of  clearness,  as  well  as  for  another  reason  which  will 
presently  be  shown,  it  is  very  useful  to  do  so.  For 
example,  we  may  write  the  reaction  we  have  been  dis- 
cussing thus: 

Carbon  dioxide  =  Oxygen  +  Carbon 

Now,  in  order  to  discover  just  how  useful  this  way 
of  writing  reactions  may  be,  and  especially  to  learn 
what  they  really  mean,  let  us  perform  and  then  con- 
sider a  carefully  conducted  chemical  experiment.  In 
performing  this  experiment,  keep  the  eyes  and  face 
away,  just  as  though  you  were  setting  off  flash  powder. 

Experiment  i.  Upon  a  small  piece  of  thin  white 
porcelain  place  a  very  little  powdered  magnesium  taken 
from  a  box  of  flashlight  powder.  Spread  the  magnesium 
out  in  a  thin  layer,  and,  supporting  the  porcelain 
suitably,  cautiously  apply  heat  to  its  lower  side  with 
an  alcohol  lamp  or  a  gas  burner,  until  all  action  ceases. 
Then  remove  the  source  of  heat,  let  the  porcelain  cool, 
and  examine  the  substance  which  it  now  holds. 

If  we  have  made  any  adequate  use  of  our  powers  of 
observation,  we  shall  have  noted  in  the  first  place  that 
the  magnesium,  which  by  the  way  is  one  of  the  elements, 
is  a  silvery  white  substance,  noticeably  light  in  weight, 
or,  as  we  should  rather  put  it,  of  low  density,  and 
metallic  looking.  In  the  second  place,  when  it  was 
heated  sufficiently  it  began  to  melt;  but  in  the  third, 
before  very  much  of  it  was  able  to  fuse,  it  took  fire  and 
gave  out  a  bright  light  together  with  a  puff  of  white 
smoke.  And,  finally,  after  cooling  it,  we  have  found 


CHEMICAL  REACTION  13 

upon  the  porcelain,  in  place  of  the  original  magnesium, 
a  white  powder  which  bears  no  resemblance  at  all  to 
the  material  with  which  we  started.  What  is  it  that  has 
happened?  Generally  we  shall  find  that  restating  such 
a  proposition  will  help  to  make  it  clearer.  We  have 
taken  the  metallic  element,  magnesium,  and  added 
heat  to  it  in  the  presence  of  the  air ;  and  have  thereby 
obtained  more  heat,  a  bright  light,  and  some  white 
powder.  This  white  powder,  a  chemical  analysis  would 
tell  us,  contains  nothing  but  magnesium  and  oxygen, 
and  the  chemist  therefore  calls  it  magnesium  oxide. 
No  matter  how  many  times  we  may  repeat  this  ex- 
periment, we  shall  always  find  at  its  conclusion  the 
same  sort  of  residue,  namely,  magnesium  oxide.  And, 
furthermore,  if  we  had  more  experience  and  much  more 
elaborate  apparatus,  we  might  weigh  the  magnesium 
beforehand  and  then,  performing  the  experiment  with 
very  great  care,  weigh  the  magnesium  oxide  produced. 
If  we  were  to  do  this  a  number  of  times  with  different 
amounts  of  magnesium,  we  should  fingl  that,  allowing 
properly  for  what  are  called  the  experimental  errors, 
the  weights  of  magnesium  oxide  produced  would  all 
be  in  the  same  proportion  to  the  weights  of  magnesium 
taken  to  start  with,  and  this  ratio  would  be  40.32  of 
magnesium  oxide  to  16  of  oxygen.  In  other  words,  when 
a  definite  weight  of  magnesium  is  heated  in  the  air,  it 
takes  from  the  air  a  definite  weight  of  oxygen  and  forms 
a  definite  weight  of  magnesium  oxide.  So,  then,  we  may 
write  this  reaction  as  an  equation: 

Magnesium  (24.32)  +  Oxygen  (16)  +  Heat 

=  Magnesium  oxide  (40.32)  +  Heat  +  Light 

By  using  other  elements  and  substances  than  mag- 
nesium and  oxygen,  it  is  found  that  the  same  rule  of 


14     CHEMISTRY  FOR  PHOTOGRAPHERS 

definite  parts  by  weight  holds  true  for  all,  although,  of 
course,  the  numerical  ratios  are  different.  Therefore  by 
our  experiment,  together  with  the  considerations  just 
developed,  it  has  been  shown,  first,  that  each  definite 
chemical  compound  always  consists  of  the  same 
elements  combined  in  the  same  proportions;  and, 
second,  that  when  two  substances  combine  chemically 
they  always  combine  according  to  definite  proportions 
by  weight.  These  are  two  fundamental  chemical  laws, 
the  law  of  constancy  of  composition  and  the  law  of 
definite  proportions,  and  they  are  of  great  importance 
because  from  these  known  ratios  and  proportions  there 
can  always  easily  be  calculated,  not  only  the  per- 
centage composition  of  substances,  but  also  either  the 
amounts  of  combining  substances  to  use  in  order  to 
get  a  particular  quantity  of  the  compound,  or  what 
quantity  of  a  compound  can  be  made  from  given 
amounts  of  its  constituents.  Thus,  from  our  equation 
and  the  ratios  given,  the  proportion  of  magnesium  in 
magnesium  oxide  is  24. 3  2^40. 3  2 ;  of  oxygen  is  1 6-^-40. 3  2 ; 
and  to  make  ten  pounds  of  magnesium  oxide  will  re- 
quire the  burning  of  10  x  24.32-^40.32  pounds  of  mag- 
nesium. 

We  will  next  make  an  experiment  by  applying  heat 
to  a  compound. 

Experiment  2.  Crush  to  a  fine  powder  a  few  small 
crystals  of  silver  nitrate,  make  a  little  pile  of  the  powder 
upon  a  clean  piece  of  thin  white  porcelain,  and  heat  it 
carefully  from  below.  Continue  to  heat  until  all  fuming 
has  ceased  and  the  residue  is  bright  and  silvery.  Cool 
and  examine  this  residue. 

Here  we  start  with  a  white  powder,  and  by  heating 
it  convert  it  to  a  bright  metal,  evidently  silver.  In  the 


CHEMICAL  REACTION  15 

course  of  the  heating,  brown  fumes  are  given  off,  the 
material  is  hot  simply  on  account  of  the  heat  we  are 
applying  to  it,  and  no  light  is  given  out.;  So  this  re- 
action appears  to  be  in  every  way  the  reverse  of  that 
in  the  first  experiment.  Before  we  can  write  out  an 
equation,  it  is  necessary  to  know  positively  whether  the 
silver  residue  and  the  brown  smoke,  which  is  an  oxide 
of  nitrogen,  are  the  only  products  of  the  reaction.  By 
putting  a  little  silver  nitrate  in  the  bottom  of  a  glass 
tube,  closed  at  one  end  and  ope/i  at  the  other,  and 
heating  it,  and  presently  applying  a  glowing  splinter  of 
wood  at  the  tube's  mouth,  we  can  show  that  oxygen  is 
also  given  off.  Finally,  by  carefully  preparing  pure 
silver  nitrate  in  the  first  place  and  weighing  it  and  then 
weighing  the  residue  of  silver,  we  should  find  that, 
just  as  there  is  a  definite  ratio  between  magnesium  and 
magnesium  oxide,  so  there  is  a  fixed  proportion  by 
weight  between  the  silver  and  the  silver  nitrate  from 
which  it  came.  This  ratio  is  i6q.8q  silver  nitrate  to 
107.88  silver.  We  are  now  in  position  to  write  the 
reaction  in  the  form  of  an  equation  if  we  add  that  it  can 
be  shown  by  suitable  means  that  fixed  proportions  of 
oxygen  and  of  nitrogen  oxide  are  also  produced  : 

Silver  nitrate  (i6q.8q)  +  Heat 

=  Silver  (107.88)  +  Oxygen  -f  Nitrogen  oxide 

Leaving  the  further  comparison  of  this  reaction  with 
that  of  Experiment  i  for  our  later  consideration,  we  will 
pass  at  once  to  two  others. 

^  Experiment  3.  Measure  in  a  test  tube  with  a  small 
glass  graduate  5  cubic  centimeters  of  water.  Weigh 
out  upon  the  balance  o.i  gram  of  silver  nitrate  and 
dissolve  it  in  the  water.  In  a  similar  manner  make 
another  solution  in  a  test  tube  of  0.3  gram  of  sodium 


1 6     CHEMISTRY  FOR  PHOTOGRAPHERS 

chloride  (common  salt)  in  10  cubic  centimeters  of 
water.  Warm  the  silver  nitrate  solution,  add  to  it  a 
little  of  the  sodium  chloride  solution,  and  shake  the 
tube  containing  the  mixture.  Add  more  of  the  sodium 
chloride  and  shake,  and  continue  thus  until  it  is  seen 
that  no  more  white  solid  forms  upon  the  addition  of  a 
drop  or  two  of  sodium  chloride  solution.  If  the  ex- 
periment has  been  carefully  done,  the  white  solid,  or 
"precipitate"  as  it  is  called,  will  collect  in  little  bunches 
and  settle  quickly  to  the  bottom  of  the  tube.  If  not, 
shake  vigorously,  and  then  let  the  tube  stand  until  all 
the  solid  has  settled,  which  should  take  only  a  few 
minutes.  Now,  remembering  that  a  little  of  the  sodium 
chloride  was  added  after  the  white  powder  ceased  to 
separate  from  the  silver  nitrate  solution,  consider  how 
to  free  the  solid  substance  from  this  sodium  chloride  and 
any  other  substance  that  may  be  in  solution  in  the 
water.  Suppose  that,  taking  care  to  disturb  as  little  as 
possible  the  solid  in  the  bottom  of  the  tube,  we  pour  off 
from  it  all  of  the  liquid  we  can,  and  then  put  10  cubic 
centimeters  more  of  fresh  water  into  the  tube.  Shake 
the  tube,  let  the  powder  settle  again,  and  then  pour 
away  the  water.  By  sufficiently  often  repeating  this 
process,  which  is  called  "washing  by  decantation,"  it  is 
clear  that  each  time  we  shall  leave  in  the  tube  only  a 
portion  of  whatever  substances  may  be  in  solution,  and 
thus  may  reduce  the  amount  of  them  to  as  small  a 
quantity  as  we  please. 

We  may  test  the  efficiency  of  this  washing  process  as 
follows:  Dissolve  in  4  or  5  cubic  centimeters  of  water 
contained  in  a  test  tube  a  small  crystal  of  silver  nitrate. 
Into  another  clean  test  tube  pour  the  wash  water 
from  the  tube  containing  the  original  precipitate.  Now 


CHEMICAL  REACTION  17 

add  to  this  wash  water  a  few  drops  only  of  the  new 
silver  nitrate  solution.  If  the  wash  water  still  contains 
an  appreciable  amount  of  sodium  chloride,  a  white 
cloudiness  will  appear,  just  as  the  white  precipitate 
was  formed  in  the  first  instance.  As  soon  as  a  portion 
of  wash  water  shows  no  perceptible  change  upon  test- 
ing with  silver  nitrate  in  this  way,  we  may  consider  the 
precipitate  thoroughly  washed. 

(NOTE. —  It  should  be  remarked  here  that  other 
soluble  chlorides  besides  the  chloride  of  sodium  will 
also  form  this  white  precipitate  in  a  solution  of  silver 
nitrate.  Since  in  some  localities  the  natural  waters 
contain  appreciable  amounts  of  chlorides  in  solution,  the 
addition  to  them  of  silver  nitrate  will  produce  a  cloudi- 
ness as  above  described.  Thus  with  such  waters  it  will 
be  impossible  thoroughly  to  wash  the  precipitate.  In 
these  cases  distilled  water  should  be  employed  for  this 
experiment.) 

In  performing  this  experiment  we  shall  have  ob- 
served that  in  the  case  of  both  substances  the  solid 
readily  dissolved  in  the  water  without  any  other 
perceptible  change ;  but  that  a  change  occurred  as  soon 
as  the  two  solutions  were  mixed ;  and  lastly  that  after  a 
certain  point  the  addition  of  sodium  chloride  failed  to 
show  any  further  change.  It  might  be  supposed  that 
the  sodium  chloride  caused  the  silver  nitrate  to  come 
out  of  the  solution  as  silver  nitrate,  or,  vice  versa,  that 
the  sodium  chloride  was  forced  out,  in  which  cases  the 
change  might  be  a  physical  change.  But  investigation 
of  the  white  precipitate  would  show  that  whereas  it 
contains  silver  it  contains  also  chlorine;  and  that  it 
contains  nothing  besides  these  two  elements.  It  is  thus 
silver  chloride,  and  therefore  the  formation  of  the  white 


1 8     CHEMISTRY  FOR  PHOTOGRAPHERS 

precipitate  indicated  a  chemical  change.  We  must  next 
consider  what  has  become  of  the  sodium  from  the 
sodium  chloride  and  the  nitrate  part  of  the  original 
silver  nitrate.  If  we  have  in  the  first  instance  added 
but  a  drop  or  two  more  of  sodium  chloride  after  the 
silver  chloride  stopped  precipitating,  we  could  take 
the  solution  decanted  from  this  precipitate  and  boil  it 
down  carefully  to  small  bulk  in  a  little  dish  and  set  it 
at  one  side  for  a  day  or  so.  We  should  then  find  that  a 
white  solid  substance  had  crystallized  out  and  examina- 
tion, perhaps  with  a  magnifier,  would  show  that  the 
crystals  have  a  different  shape  from  either  sodium 
chloride  or  silver  nitrate  crystals.  Also  by  conducting 
the  experiment  quantitatively,  adding  to  a  known 
amount  of  silver  nitrate  a  definite  quantity  of  sodium 
chloride  in  excess,  and  weighing  the  silver  chloride 
precipitated,  it  can  easily  be  proved  that  besides  the 
silver  chloride  the  only  other  new  substance  formed  is 
sodium  nitrate.  Therefore  it  should  be  noted  that  we 
have  also  indicated  by  the  experiment  that  when 
silver  chloride  is  formed  by  interaction  of  silver  nitrate 
and  sodium  chloride,  exactly  the  right  amount  of 
sodium  is  involved  to  form  sodium  nitrate  with  the 
nitrate  from  the  silver  compound.  There  is  neither 
nitrate  nor  sodium  left  over,  so  to  speak.  Looking  at 
the  matter  in  this  way,  we  may  see  that  any  definite 
quantity  of  silver  nitrate  is  perfectly  matched  by  some 
other  definite  quantity  of  sodium  chloride,  because, 
while  there  is  just  enough  and  no  more  silver  in  the  one 
to  combine  with  all  the  chloride  in  the  other,  and  form 
silver  chloride,  at  the  same  time  there  are  exactly 
suitable  amounts  present  of  sodium  and  nitrate  to  form 
sodium  nitrate  and  have  nothing  left  over.  Such  amounts 


CHEMICAL  REACTION  ig 

of  two  interacting  substances  are  called  equivalent 
amounts,  or  simply  equivalents.  We  are  now  able  to 
write  the  reaction  as  an  equation : 

Silver  nitrate  (169.89)  +  Sodium  chloride  (58.46) 
=  Silver  chloride  (143.34)  +  Sodium  nitrate  (85.01) 

To  keep  the  matter  fresh  in  our  minds  we  will  note 
again  that  by  using  the  proportions  of  these  numbers, 
which  we  now  will  call  the  equivalents  of  the  sub- 
stances, it  is  possible  to  calculate  from  a  given  weight 
of  any  one  of  the  compounds  the  corresponding  weights 
of  all  the  others. 

Experiment  4.  Dilute  with  water  a  few  cubic  centi- 
meters of  strong  hydrochloric  acid  (muriatic  acid)  to  25 
cubic  centimeters,  mix  thoroughly,  and  put  20  cubic 
centimeters  of  the  solution  into  a  small  porcelain  dish, 
saving  the  remainder,  5  cubic  centimeters,  in  a  test 
tube.  To  5  cubic  centimeters  of  water  contained  in 
another  test  tube  add  two  or  three  drops  of  strong 
ammonium  hydroxide  (ammonia  water)  and  mix  by 
shaking.  Dip  into  the  smaller  portion  of  diluted 
hydrochloric  acid  one  end  of  a  strip  of  red  litmus  paper, 
and  similarly  a  strip  of  blue  litmus  paper,  and  observe 
the  effects.  In  the  same  way  try  the  action  of  the  diluted 
ammonium  hydroxide  upon  red  and  upon  blue  litmus, 
and  compare  these  effects  with  the  former.  Evidently 
the  litmus  can  be  used  to  indicate  the  presence  of  these 
substances  in  solution,  and  for  this  reason  it  is  called  an 
"indicator." 

To  the  bulk  of  the  hydrochloric  acid  solution  in  the 
dish  add  cautiously  strong  ammonium  hydroxide,  little 
by  little,  stirring  after  each  addition  to  mix  the  liquids. 
Continue  this  operation  until  the  mixture  smells  very 
faintly  of  ammonia.  Try  the  effect  of  the  solution  now 


io      CHEMISTRY  FOR  PHOTOGRAPHERS 

upon  red  litmus,  and  consider  what  has  happened  to  the 
hydrochloric  acid  in  the  operation.  Leaving  the  strip 
of  litmus  paper  in  the  liquid,  cautiously  add,  a  drop  or 
two  at  a  time,  some  of  the  diluted  hydrochloric  acid 
from  the  test  tube  until,  upon  thorough  stirring,  the 
litmus  just  turns  red.  Then  add  with  still  greater  care 
a  very  little  diluted  ammonium  hydroxide,  continuing 
thus  with  one  or  the  other  of  these  dilute  solutions  until, 
if  possible,  red  litmus  remains  red  and  blue  remains 
blue,  or  at  any  rate  the  color  change  is  very  slow.  After 
the  removal  of  all  strips  of  paper,  the  dish  is  to  be  set 
over  a  burner  in  such  a  way  that  all  the  water  may  be 
evaporated  slowly,  the  dish  at  no  time  being  permitted 
to  become  hotter  than  the  boiling  temperature.  Ex- 
amine the  dish  for  any  residue.  Boil  away  a  little  of 
each  of  the  dilute  solutions  separately  and  see  whether 
any  residue  is  left  in  either  case. 

(NOTE. — If,  after  all  the  water  has  been  evaporated, 
the  dish  is  heated  much  above  the  boiling  temperature, 
the  experiment  will  probably  be  spoiled,  since  the 
residue  also  will  be  expelled.  It  is  quite  likely  that  both 
the  liquid  and  the  residue  may  be  slightly  colored  from 
the  litmus,  if  the  strips  of  paper  have  been  allowed  to 
soak  in  the  dish  for  any  length  of  time.  Keep  the  litmus 
out  of  the  solution  as  much  as  possible,  and  the  residue 
will  be  nearly,  if  not  quite,  white  at  the  end  of  the 
experiment.) 

Remarking  that  "hydrochloric  acid"  as  supplied  is 
merely  a  solution  of  hydrogen  chloride  in  water,  and 
that  ammonium  hydroxide  is  likewise  a  solution  of 
ammonia,  both  these  compounds  being  gaseous,  we 
may,  if  we  look  at  the  equation  which  represents  the 


CHEMICAL  REACTION  21 

reaction  of  Experiment  3,  perhaps  at  once  venture  to 
write  an  equation  for  this  reaction,  as  follows : 

Ammonium  hydroxide  -f  Hydrogen  chloride 

=  Ammonium  chloride  +  Hydrogen  hydroxide 

The  white  residue  is  therefore  ammonium  chloride, 
since  hydrogen  hydroxide  is  only  a  chemical  name  for 
water.  This  reaction  is  a  very  remarkable  one,  in  spite 
of  its  apparent  simplicity.  Two  clear,  colorless  liquids, 
neither  one  of  which  upon  evaporation  leaves  any 
residue,  the  one  smelling  strongly  of  ammonia  and  the 
other  producing  a  pungent,  stinging  sensation  in  the 
nostrils,  when  suitably  mixed  form  a  solution  which 
has  practically  no  odor  at  all,  from  which  a  white  solid 
substance  is  obtained  by  evaporating  the  water.  It  is 
also  to  be  noted  that  the  two  substances,  ammonium 
hydroxide  and  hydrogen  chloride,  interact  spontane- 
ously and  without  the  intervention  of  heat,  just  as  did 
the  silver  nitrate  and  sodium  chloride  of  the  preceding 
experiment;  that  heat  is  developed  by  reaction,  as 
could  be  shown  by  using  stronger  solutions  of  the 
substances;  and,  as  in  the  foregoing  experiments, 
that  if  the  quantities  of  interacting  substances  and 
products  were  measured  they  would  be  found  in  definite 
proportions  to  each  other.  So  we  will  write  the  equation 
again  in  the  following  form : 

Ammonium  hydroxide  (35.05)  -f  Hydrogen  chloride  (36.48) 
=  Ammonium  chloride  (53.502)  +  Water  (18.016)  +  Heat 

Since,  as  we  have  seen,  the  two  odors  mutually  destroy 
each  other,  it  is  but  a  step  to  the  inference  that  the 
ammonium  hydroxide  and  hydrochloric  acid  have 
neutralized  each  other.  Indeed,  there  is  a  whole  series 
of  substances  that  possess,  like  the  acid,  a  sharp,  sour 
taste  when  sufficiently  dilute,  are  generally  powerful, 


22  CHEMISTRY  FOR  PHOTOGRAPHERS 

corrosive  liquids  in  concentrated  form,  and  have 
certain  other  important  properties  in  common.  And 
there  is  another  series,  like  the  ammonium  hydroxide, 
possessing  in  dilute  solution  a  soapy  taste,  generally 
solids  forming  more  or  less  caustic  solutions.  And  the 
members  of  the  first  series  interact  with  those  of  the 
second  series  in  the  same  way  as  the  hydrochloric  acid 
and  ammonium  hydroxide.  The  first  are  called  acids 
and  the  second,  bases.  The  process  of  their  interaction, 
which  is  always  accompanied  by  the  development  of 
heat,  is  called  neutralization,  and  the  principal  product 
is  a  "salt"  in  each  case,  the  other  product  always  being 
water.  So  when  a  given  solution  contains  an  excess  of 
acid,  or  free  acid  as  it  is  called,  if  the  excess  is  un- 
desirable it  may  be  readily  destroyed  by  the  addition  of 
a  sufficient  amount  of  a  base,  or  vice  versa.  Am- 
monium hydroxide  is  often  used  for  getting  rid  of  an 
excess  of  acid  because  it  is  itself  volatile,  and  if  an  excess 
of  alkali  is  also  not  wanted  it  can  be  expelled  by  heating. 
In  these  four  experiments  we  have  learned  that 
elements  will  sometimes  unite  to  form  compounds  if 
sufficient  heat  energy  be  supplied  to  them;  that  com- 
pounds may  be  caused  by  heat  to  break  apart  into  the 
elements  of  which  they  consist;  and  that  often  when 
chemical  reactions  occur  there  is  a  development  of 
heat,  and  sometimes  even  of  light.  Sometimes,  as  in 
the  decomposition  of  silver  nitrate  in  the  second 
experiment,  the  reaction  does  not  yield  heat,  but  on 
the  other  hand  heat  disappears,  being  used  up  in  the 
progress  of  the  reaction.  Thus  every  chemical  reaction 
is  accompanied  by  an  energy  change  of  some  sort. 
Even  when,  as  in  the  neutralization  of  a  base  by  an 
acid,  the  reaction  starts  spontaneously,  that  there  is 


CHEMICAL  REACTION  23 

actually  a  change  in  the  energy  conditions  is  proved  by 
the  appearance  of  heat  when  the  base  and  acid  are 
mixed.  This  fact  of  a  relation  between  energy  and 
chemical  reaction  is  of  the  greatest  importance  in  very 
many  ways  which  need  not  here  be  discussed.  But  in 
the  chemistry  of  photography  it  is  vital,  for  it  is  the 
reaction  produced  in  the  photographic  emulsion  by  the 
energy  of  light  which  makes  photography  possible. 

By  a  comparison  of  the  experiments  we  may  also 
learn  that  there  are  several  different  types  of  reactions. 
When  two  or  more  elements  or  substances  are  caused 
to  combine,  as  in  the  burning  of  magnesium  with 
oxygen  to  magnesium  oxide,  the  elements  are  put 
together,  and  the  reaction  is  thus  a  reaction  of  synthesis. 
On  the  other  hand,  it  is  possible,  as  in  the  second 
experiment,  to  cause  compounds  to  be  broken  up  into 
their  constituents,  and  then  we  have  reactions  of 
analysis.  It  is  the  use  of  this  kind  of  reaction,  whereby 
the  composition  of  substances  is  investigated  and  their 
percentage  composition  determined,  which  constitutes 
that  branch  of  chemical  science  known  as  analytical 
chemistry.  In  such  reactions  as  that  between  silver 
nitrate  and  sodium  chloride  and  between  hydrochloric 
acid  and  ammonium  hydroxide,  we  appear  to  have 
compounds  picked  apart  and  put  together  again  in  a 
different  order.  These  reactions,  in  which  there  is  a 
change  of  partners  so  to  speak,  are  reactions  of  me- 
tathesis. But  there  are  still  other  ways  in  which  re- 
actions may  be  classified,  for  example,  we  have  already 
shown  that  in  Experiment  4  the  process  of  neutraliza- 
tion is  illustrated.  When  the  dry  substances  are  mixed 
together  and  reaction  is  brought  about  either  without 
or  with  the  aid  of  heat,  it  is  sometimes  said  that  the 


24     CHEMISTRY  FOR  PHOTOGRAPHERS 

substances  react  "in  the  dry  way;"  and  the  reaction 
may  be  said  to  take  place  "in  the  wet  way"  when  it 
occurs  between  substances  that  are  in  solution.  The 
most  important  kinds  of  reaction  for  our  discussion, 
however,  are  typified  in  the  first  two  experiments,  since 
it  is  by  means  of  such  reactions  as  these  that  the  initial 
work  of  light  upon  the  photographic  plate  or  film  is 
afterward  made  effective  in  the  negative  (or  print) 
and  the  result  finally  modified  to  suit  the  requirements 
of  the  photographer. 

When,  as  in  Experiment  i ,  oxygen  is  caused  to  com- 
bine with  a  substance,  that  substance  is  said  to  be 
oxidized,  and  the  reaction  is  called  oxidation.  The 
oxygen  may  come,  as  in  the  experiment,  from  the 
supply  of  free  oxygen  gas  in  the  air,  or  it  may  be  taken 
from  (or  given  up  by,  whichever  way  we  choose  to 
look  at  it)  some  other  compound,  generally  one  that  is 
rich  in  oxygen.  The  substance  which  furnishes  the 
oxygen  is  called  an  oxidizing  agent  or  simply  an  oxidizer. 
If  we  now  consider  what  has  happened  to  the  oxidizer, 
namely  that  oxygen  has  been  taken  away  from  it,  we 
shall  see  that  this  is  precisely  what  has  occurred  in 
heating  the  silver  nitrate,  viz.,  oxygen  has  been  re- 
moved. This  is  evidently  the  reverse  process  to  oxida- 
tion, and  it  is  called  reduction.  The  substance,  as  for 
instance  silver  nitrate,  from  which  oxygen  is  taken 
is  said  to  be  reduced ;  and  that  which  takes  the  oxygen 
is  called  a  reducer.  It  may  now  be  remarked  that  a 
reduction  cannot  occur  without  an  accompanying  and 
equivalent  oxidation,  and  vice  versa.  The  substance 
which  is  oxidized  need  not  be  an  element,  it  may  even 
already  be  an  oxide.  As  an  example,  there  is  an  oxide 
of  lead,  litharge,  which  is  capable  of  being  oxidized, 


CHEMICAL  REACTION  15 

forming  another  oxide  of  lead,  richer  in  oxygen,  lead 
dioxide,  or,  as  it  is  sometimes  called,  lead  peroxide.  This 
lead  peroxide  is  a  strong  oxidizing  agent,  readily  giving 
up  again  exactly  the  amount  of  oxygen  which  was 
required  to  change  it  from  litharge  to  peroxide.  After 
the  peroxide  has  been  reduced  to  litharge,  the  latter 
can  be  further  reduced  to  metallic  lead,  by  using  still 
more  powerful  reducers.  Reduction,  therefore,  may 
also  be  conducted  in  steps,  a  higher  oxide  being  re- 
duced to  a  lower,  and  the  lower  completely  reduced,  as 
to  a  metal.  By  an  extension  of  these  principles,  chemical 
changes  in  which  no  oxygen  at  all  is  involved  are 
denominated  oxidation  and  reduction.  As  an  example 
may  be  cited  the  action  of  a  piece  of  iron,  such  as  a 
nail,  on  a  boiling  solution  of  the  chloride  of  iron  which 
is  called  ferric  chloride.  The  solution  is  originally 
yellow  in  color,  but  the  boiling  over  metallic  iron 
bleaches  out  the  yellow  and  makes  the  solution  color- 
less. At  the  same  time  a  little  of  the  metallic  iron  is 
dissolved.  By  suitable  analytical  tests  it  can  be  shown 
that  the  solution  now  contains  chloride  of  iron  and 
nothing  else,  but  the  iron  chloride  is  a  different  one 
from  the  original.  If  it  were  taken  out  of  the  solution 
it  would  be  found  to  be  a  white  powder,  whereas  the 
ferric  chloride  is  yellow.  The  new  compound  is  ferrous 
chloride,  and  it  is  made  from  the  ferric  compound 
by  reduction,  iron  being  the  reducing  agent.  It  will 
perhaps  be  clear  at  once  that  the  iron  metal,  from  the 
nail,  which  goes  into  solution  in  the  process  is  oxidized, 
so  that  the  quantity  of  ferrous  chloride  finally  present 
comes  not  only  from  the  reduction  of  ferric  chloride  to 
ferrous  chloride,  but  partly  also  from  the  oxidation  of 
iron  to  ferrous  chloride.  Ferrous  chloride  can  finally  be 


26     CHEMISTRY  FOR  PHOTOGRAPHERS 

oxidized  back  to  ferric  chloride  by  passing  chlorine  gas 
into  the  solution.  Thus  in  these  oxidations  and  reduc- 
tions we  have  nothing  whatever  to  do  with  the  element 
oxygen,  but  have  learned  that  chlorine  is  an  effective 
oxidizing  agent,  and  that  reduction  may  involve  the  re- 
moval of  other  elements  than  oxygen. 

(NOTE. — For  a  more  complete  account  of  this  sub- 
ject the  reader  is  referred  to  any  of  the  standard  works 
on  chemistry.  I  twill  be,  in  fact,  an  excellent  idea  if  he 
will  extend  his  reading  in  these  chemistries  to  cover 
such  subjects  as  valency,  the  ionic  theory,  and  molec- 
ular and  atomic  weights,  which  are  not  within  the 
scope  of  the  present  work.) 


CHAPTER  III 
LIGHT  AND  CHEMICAL  REACTION 


WE  have  learned  that  some  chemical  reactions 
require,  in  order  to  start,  nothing  more  than  the 
mere  contact  of  the  substances  involved,  at  the  ordinary 
temperatures.  On  the  other  hand,  of  those  reactions 
which  need  to  be  initiated  by  the  application  of  energy 
to  the  reacting  factors,  there  are  two  kinds :  first,  such 
reactions  as,  when  once  started,  develop  greater  or  less 
quantities  of  heat  energy  by  means  of  which  they  are 
enabled  to  proceed  to  a  finish,  one  or  other,  or  some- 
times both,  of  the  factors  being  used  up;  and,  second, 
those  which  continue  only  so  long  as  sufficient  energy 
is  applied,  the  energy  being  absorbed  in  the  process. 
The  terms  energy,  and  heat  energy,  are  here  employed 
in  the  most  general  sense  to  include  all  those  vibrations 
of  the  ether,  whereby  radiant  energy  is  transmitted 
through  space,  and  without  reference  to  the  wave- 
length of  the  vibrations.  In  the  present  chapter  we 
shall  limit  ourselves  to  a  consideration  of  the  energy 
of  the  shorter  wave-lengths,  and  illustrate  and  discuss 
some  typical  chemical  reactions  which  are  originated  by 
the  energy  of  light,  since  it  is  these  photo-chemical 
reactions,  as  they  are  called,  which  are  of  primary  con- 
sequence in  the  making  of  pictures  by  photographic 
methods. 

That  reactions  of  this  kind  are  continually  going  on 
in  nature  will  readily  be  seen  if  we  merely  mention  the 
tanning  of  the  skin  by  exposure  to  sunshine,  the  fading 
of  colors  in  bright  light,  and  the  process  referred  to  in 

27 


28   CHEMISTRY  FOR  PHOTOGRAPHERS 

the  preceding  chapter  by  which  chlorophyll  of  the  plant 
world,  absorbing  the  energy  of  light  and  utilizing  it, 
decomposes  the  carbon  dioxide  of  the  air.  Perhaps  we 
may  get  some  idea  of  the  amount  of  work  involved  in 
this  operation,  if  we  stop  to  consider  how  great  a 
quantity  of  heat  is  given  out  when  wood  is  burned, 
remembering  that  the  heat  energy  set  free  in  the  burn- 
ing of  each  pound  of  carbon  to  form  carbon  dioxide 
is  exactly  equivalent  to  the  energy  absorbed  in  the 
separation  of  this  amount  of  carbon.  But  the  sub- 
stances concerned  in  these  photo-chemical  reactions 
are  too  complex  and  the  changes  too  complicated  for 
our  present  consideration.  It  will  be  well,  therefore, 
for  us  to  begin  with  something  simpler  and  more  easily 
comprehended. 

Certain  of  the  elerpents  are  affected  in  a  very  peculiar 
way^bylight.  For  example,  the  common  form  of  sul- 
phur/rolr  sulphur,  or  brimstone,  occurs  in  rhombic  crys- 
tals and  is  very  soluble  in  carbon  disulphide.  But  upon 
exposure  to  light  it  is  changed  to  an  amorphous  condi- 
tion (non-crystalline)  in  which  it  is  no  longer  soluble  in 
the  solvent  mentioned,  and  at  the  same  time  heat  is 
given  out : 

Sulphur  (rhombic)  +  Light  (Violet) 

=  Sulphur  (amorphous)  +  Heat  (Infra-red) 

The  element  phosphorus  goes  through  a  similar  change 
in  which  the  two  varieties  are  even  more  differentiated 
from  each  other.  Phosphorus  may  be  purchased  either 
in  waxy,  yellowish-white  sticks  or  as  a  red  powder. 
Yellow  phosphorus  melts  at  44.4  degrees  centigrade, 
is  soluble  in  carbon  disulphide,  and  is  extremely  poison- 
ous. Matches  were  formerly  manufactured  with  this 
yellow  form,  and  workmen  in  match  factories  were 


LIGHT  AND  CHEMICAL  REACTION       19 

subject  to  a  peculiarly  dreadful  disease.  When  the 
yellow  form  is  exposed  to  light,  particularly  the  blue, 
violet,  and  ultra-violet  rays,  it  is  thereby  converted  to 
the  red  modification,  with  the  liberation  of  heat.  Red 
phosphorus  does  not  melt  even  at  red  heat,  is  insoluble 
in  carbon  disulphide,  and  is  not  poisonous.  After  much 
vigorous  objection  by  the  match  manufacturers  be- 
cause of  the  greater  cost  of  red  over  yellow  phosphorus, 
laws  were  passed  several  years  ago  prohibiting  the  use 
of  the  poisonous  yellow  form,  and  matches  are  now  made 
out  of  the  harmless  red  variety : 

Phosphorus  (yellow)  +  Light  =  Phosphorus  (red)  +  Heat 
Selenium  is  another  element  which  is  light  sensitive. 
It  can  be  prepared  as  a  bright  red,  amorphous  powder 
which  is  soluble  in  carbon  disulphide,  melts  at  125 
degrees  centigrade,  and  is  a  non-conductor  of  electricity. 
By  the  action  of  violet  light,  red  selenium  is  changed  to 
a  dark  gray,  crystalline,  metallic  form.  This  is  insoluble 
in  carbon  disulphide,  melts  at  217  degrees  centigrade, 
and  is  a  variable  conductor  of  the  electric  current 
according  to  the  intensity  of  light  thrown  upon  it : 

Selenium  (amorphous)  +  Light 

=  Selenium  (metallic)  +  Heat 

Several  interesting  uses  have  been  made  of  the  peculiar 
property  of  the  metallic  selenium,  namely,  its  varia- 
bility in  conductivity  produced  by  changes  in  the 
illumination.  The  resistance  of  a  "selenium  cell"  may 
be  as  much  as  two  hundred  times  as  great  in  the  dark 
as  in  the  light.  By  use  of  such  cells  in  electrical  circuits, 
motors  can  be  started  and  stopped  by  a  beam  of  light, 
or  signal  lights  caused  to  operate  automatically,  the 
gas  or  current  being  turned  on  by  the  coming  of  dark- 
ness and  turned  off  again  with  the  daylight.  Perhaps 


30  CHEMISTRY  FOR  PHOTOGRAPHERS 

the  most  wonderful  application  of  this  light  reaction 
is  in  the  transmission  of  photographs  by  telegraph. 

The  three  photo-chemical  reactions  just  described  are 
evidently  quite  different  in  their  nature  from  such  a 
reaction,  for  example,  as  the  reduction  of  ferric  to  fer- 
rous chloride,  which  we  shall  presently  consider.  In 
some  so  far  unexplained  way  they  appear  to  be  read- 
justments in  the  energy  conditions  prevailing  in  the 
molecules  of  these  light-sensitive  elements. 

If  precisely  equal  volumes,  measured  under  identical 
conditions  of  temperature  and  pressure,  of  the  two 
gaseous  elements  hydrogen  and  chlorine  are  put  into 
a  glass  jar  and  kept  in  total  darkness,  the  amount  of 
hydrogen  chloride  formed  in  a  given  time  by  the 
chemical  union  of  these  elements  will  be  inappreciable. 
Practically  we  may  say  that  in  the  absence  of  light  no 
reaction  occurs.  When,  however,  the  jar  is  brought  out 
and  set  in  a  very  weak  light,  the  hydrogen  and  chlorine 
begin  slowly  to  combine,  and  after  the  lapse  of  sufficient 
time  there  will  be  found  in  the  jar  only  hydrogen 
chloride  gas.  If  the  jar  be  put  in  a  more  brightly 
illuminated  place,  the  reaction  becomes  more  rapid. 
By  exposing  it  to  direct  sunlight,  indeed,  it  becomes  so 
much  accelerated  as  to  be  violently  explosive,  and  the 
glass  jar,  unless  very  stout,  will  probably  be  shattered. 
In  performing  this  experiment  it  is  usual  to  mix  the 
gases  in  the  dark,  wrap  the  jar  completely  in  a  black 
cloth  to  which  has  been  attached  a  long  string,  and  set 
the  apparatus  in  the  sunlight.  The  experimenter  there- 
upon retreats  to  the  other  end  of  the  cord  and  jerks 
away  the  protecting  cloth.  The  explosion  follows  in- 
stantly, and  is  occasioned  by  the  fact  that  when 
hydrogen  and  chlorine  unite  a  tremendous  evolution 


LIGHT  AND  CHEMICAL  REACTION      31 

of  heat  takes  place.  Since  the  product,  hydrogen 
chloride,  is  also  gaseous,  it  expands  with  great  pressure 
under  the  influence  of  the  liberated  heat.  It  is  needless 
to  say,  perhaps,  that  this  experiment  is  a  very  dangerous 
one.  The  reaction  is: 

Hydrogen  +  Chlorine  +  Light 

=  Hydrogen  chloride  -f  Heat 

Hydrogen  iodide  is  a  gas  similar  in  many  respects  to 
the  hydrogen  chloride  we  have  just  been  discussing. 
Its  solution  in  water  is  colorless,  and  is  commonly  called 
hydriodic  acid.  When  such  a  colorless  solution  is  ex- 
posed to  both  the  oxygen  of  the  air  and  light,  it  turns 
brown.  Light  causes  decomposition  of  the  hydrogen 
iodide,  setting  free  iodine,  which  in  solution  has  a 
brown  color,  if  water  is  the  solvent.  The  hydrogen 
combines  with  oxygen  from  the  air,  forming  water, 
so  the  reaction  may  be  expressed : 

Hydrogen  iodide  +  Oxygen  +  Light  =  Iodine  +  Water 

Blue  light  is  most  effective  in  bringing  this  reaction 
about.  But  we  must  note  in  this  case  that  without 
the  presence  of  oxygen  the  reaction  could  not  go  on,  as 
hydrogen  and  iodine  would  recombine  directly  they 
were  separated,  being  both  chemically  "active  sub- 
stances. It  would  probably  be  more  correct  to  say  that 
under  such  circumstances  both  these  actions  would  be 
going  on  simultaneously,  hydrogen  and  iodine  re- 
combining  to  form  hydrogen  iodide  as  fast  as  the  latter 
was  being  decomposed.  That  is,  we  might  write  the 
reaction  in  the  following  way : 

Hydrogen  iodide  ^J^  Hydrogen  +  Iodine 

The  direction  of  the  arrows  signifies  that  both  reac- 
tions are  proceeding  at  the  same  time.  Such  a  reaction 


3z   CHEMISTRY  FOR  PHOTOGRAPHERS 

is  said  to  be  "reversible."  But  when  oxygen  is  at  hand, 
the  hydrogen  unites  with  it  in  preference  to  iodine,  and 
thus  the  decomposition  of  hydrogen  iodide  is  enabled  to 
go  on,  reaction  progressing  only  from  left  to  right  in  the 
above  equation.  As  we  shall  see,  there  are  other  photo- 
chemical reactions  in  which  the  presence  of  some  addi- 
tional substance  is  required  in  order  that  light  may 
produce  the  effect.  Photographically  speaking,  we  should 
say  that  this  extra  substance  is  a  "sensitizer,"  the  de- 
composable material,  such,  for  example,  as  hydrogen 
iodide,  being  "insensitive"  in  the  absence  of  the  "sen- 
sitizer." 

Another  reaction  of  decomposition  brought  about  by 
light  occurs  with  ferric  oxalate,  a  compound  which  is 
of  considerable  photographic  importance.  When  dis- 
cussing the  subject  of  oxidation  and  reduction,  we 
showed  how  by  suitable  means  a  ferric  salt  is  reduced  to 
ferrous  salt,  and  the  ferrous  oxidized  back  to  ferric. 
Light  is  able  to  perform  this  Deduction  with  ferric 
oxalate,  ferrous  oxalate  being  produced,  and  carbon 
dioxide  escaping. 

Experiment  5.  Expose  to  strong  sunlight  for  half  or 
three  quarters  of  an  hour  a  few  of  the  brownish  scales 
of  ferric  oxalate,  and  at  the  expiration  of  this  time  look 
for  any  change  which  has  been  produced.  Save  the 
material  for  Experiment  7.  It  will  be  well  for  us  here  to 
learn  how  to  detect  the  presence  of  ferrous  and  ferric 
iron  in  substances,  since  in  the  use  of  ferric  oxalate  for 
photographic  purposes  it  is  very  necessary  to  have  no 
ferrous  salt  present.  If  the  ferric  oxalate  is  from  old 
stock  or  has  been  handled  much  in  the  light,  it  is  very 
likely  to  have  suffered  reduction  in  part,  and  this  will 
render  it  useless.  The  brown  color  of  the  bottle  in 


LIGHT  AND  CHEMICAL  REACTION      33 

which  it  is  bought  and  must  be  kept  is  intended  to  pro- 
tect it  from  the  action  of  all  light  except  that  trans- 
mitted by  the  brown  glass.  We  will  first  make  an 
experiment  to  show  how  ferric  salt  is  detected. 

Experiment  6.  Dissolve  in  5  cubic  centimeters  of 
water  a  small  crystal  of  ferrous  sulphate,  add  a  few 
drops  of  dilute  sulphuric  acid  and  shake  up  this  solution 
in  the  test  tube  for  several  minutes.  Since  there  is  air 
in  the  test  tube  above  the  solution  and  ferrous  salt  is 
very  readily  oxidized  by  air,  this  shaking  will  give  some 
of  the  ferrous  sulphate  in  the  solution  an  opportunity  to 
be  oxidized  to  ferric  sulphate.  We  might  do  the  same 
thing  more  quickly  by  adding  to  the  liquid  a  drop  or 
two  of  nitric  acid,  which  is  a  good  oxidizer.  In  either 
case  we  shall  have  the  same  result  in  effect,  as  it  may  be 
represented : 

Ferrous  sulphate  +  Sulphuric  acid  +  Oxygen 

=  Ferric  sulphate  +  Water 

In  the  presence  of  nitric  acid  there  would  be  formed 
also  some  ferric  nitrate. 

Next  take  a  very  small  crystal  of  potassium  ferro- 
cyanide,  or  as  the  druggist  would  probably  call  it, 
yellow  prussiate  of  potash,  break  it  into  still  smaller 
bits  and  dissolve  a  minute  piece  in  5  cubic  centimeters 
of  water.  This  will  be  a  very  dilute  solution.  In  the 
same  way  make  5  cubic  centimeters  of  a  dilute  solution 
of  potassium  ferricyanide,  or  red  prussiate  of  potash. 
Take  about  a  cubic  centimeter  of  the  prepared  solution 
containing  ferric  sulphate  in  a  clean  test  tube,  dilute 
it  with  water  to  about  5  cubic  centimeters,  and  add  a 
few  drops  of  the  dilute  potassium  ferrocyanide.  The 
fine  blue  color,  which  is  Prussian  blue,  is  due  to  the 
formation  of  ferric  ferrocyanide,  and  is  always  ob- 


34   CHEMISTRY  FOR  PHOTOGRAPHERS 

tained  when  solutions  of  potassium  ferrocyanide  and  a 
ferric  salt  are  mixed.  Thus  the  blue  coloration  is  a 
chemical  test  for  ferric  iron.  Now  boil  5  or  6  cubic 
centimeters  of  water  in  a  clean  test  tube,  to  expel  all 
oxygen  which  is  in  solution  in  the  water,  cool  without 
shaking  (to  prevent  as  much  as  possible  the  water  from 
absorbing  more  oxygen),  and  dissolve  in  it  a  very  small, 
clear  crystal  of  ferrous  sulphate.  As  quickly  as  may  be, 
add  to  it  a  few  drops  of  potassium  ferricyanide  solution. 
The  blue  color  which  appears  in  this  case  is  Turnbull's 
blue,  and  is  always  obtained  when  solutions  of  potas- 
sium ferricyanide  and  a  ferrous  salt  are  mixed,  and  is 
therefore  a  chemical  test  for  ferrous  iron. 

So,  to  repeat,  ferrocyanide  gives  blue  with  ferric  iron 
salts;  and  ferricyanide  gives  blue  with  ferrous  com- 
pounds. By  writing  out  these  reactions  in  equations  we 
shall  see  that  they  are  reactions  of  metathesis,  as  follows : 

(1)  Ferric  sulphate  +  Potassium  ferrocyanide 

=  Ferric  ferrocyanide  -f  Potassium  sulphate 

(2)  Ferrous  sulphate  +  Potassium  ferricyanide 

=  Ferrous  ferricyanide  +  Potassium  sulphate 

If  the  experiments  have  been  carefully  done  according 
to  directions,  it  will  be  apparent  that  these  tests  are 
very  delicate. 

Experiment  7.  Treat  in  a  test  tube  with  5  or  6  cubic 
centimeters  of  water  a  little  of  the  ferric  oxalate  which 
was  exposed  to  light  in  Experiment  5.  If  any  powder 
remains  undissolved,  add  a  crystal  or  two  of  potassium 
oxalate  to  get  it  into  solution.  Shake  the  tube,  and  to  its 
contents  add  a  few  drops  of  potassium  ferricyanide 
solution  prepared  in  Experiment  6.  Blue  coloration 
indicates  the  presence  of  ferrous  iron.  Compare  with 
this  the  action  of  ferricyanide  upon  a  fresh,  cold  solution 


LIGHT  AND  CHEMICAL  REACTION      35 

of  ferric  oxalate.  If  the  latter  compound  is  pure,  there 
will  be  no  blue  color  developed  in  salt  which  has  not 
been  exposed  to  light. 

The  question  whether  ferric  oxalate  contains  any 
ferrous  salt  as  an  impurity  must  be  answered  upon  the 
basis  of  what  takes  place  immediately  when  the  mix- 
ture of  the  two  solutions  is  made  and  not  upon  what  may 
happen  after  this  mixture  has  stood  for  some  time : 

Ferric  oxalate  +  Light  =  Ferrous  oxalate  +  Carbon  dioxide 

If  the  ferrous  oxalate  is  put  away  in  the  dark  in  con- 
tact with  the  air,  ferric  oxalate  forms  again  by  oxidation. 

Experiment  8.  Dissolve  a  crystal  of  uranium  nitrate 
in  5  cubic  centimeters  of  water,  spread  about  half  of 
this  solution  upon  a  piece  of  paper,  using  a  sized  paper 
such  as  writing  paper,  and  lay  it  aside  in  a  dark  place  to 
dry.  To  the  remainder  of  the  uranium  nitrate  solution 
add  a  few  drops  of  dilute  potassium  ferrocyanide,  and 
note  the  blood-red  color  produced.  As  soon  as  the 
prepared  paper  is  dry,  expose  it  to  strong  sunlight  for  a 
few  minutes  and  then  flow  over  it  a  little  of  the  potas- 
sium ferrocyanide  and  compare  the  result  with  the 
preceding. 

Uranium  salts  are  extremely  sensitive  to  light, 
uranyl  compounds  being  reduced  to  uranous.  Potas- 
sium ferrocyanide  forms  with  uranyl  nitrate  red  uranyl 
ferrocyanide.  When  exposed  to  light,  uranyl  salts  are 
reduced,  and  then  with  ferrocyanide  the  green  uranous 
ferrocyanide  is  obtained. 

Experiment  q.  Make  up  a  little  potassium  dichro- 
mate  solution,  coat  with  it  a  piece  of  sized  paper,  and 
dry  it  in  the  dark.  When  dry  expose  it  to  sunlight  in 
a  printing  frame  under  a  negative  for  five  minutes.  On 


36  CHEMISTRY  FOR  PHOTOGRAPHERS 

examination  it  will  be  seen  that  opposite  the  thin  places 
in  the  negative  the  orange  color  of  the  dichromate  has 
turned  brown,  but  beneath  the  dense  parts  there  is 
little  or  no  color  change. 

In  order  to  investigate  this  reaction  a  little  further 
and  to  find  out  if  possible  what  may  have  happened  to 
the  chromium  compound,  let  us  take  4  or  5  cubic 
centimeters  of  ferrous  sulphate  solution,  add  to  it  a  little 
of  our  solution  of  potassium  dichromate  and  then  make 
the  test  with  potassium  ferrocyanide  for  ferric  iron. 
We  shall  discover  by  this  means  that  potassium  di- 
chromate is  a  good  oxidizer,  since  it  quickly  changes 
ferrous  sulphate  to  the  ferric  salt.  Since  the  dichromate 
oxidizes  ferrous  iron,  it  must  itself  be  reduced.  Now, 
returning  to  the  paper  coated  with  dichromate,  we 
have  here  an  oxidizing  agent  in  contact  with  the 
materials  of  which  the  paper  and  sizing  consist.  These 
are  organic  substances,  principally  cellulose  in  the 
paper  and  possibly  starch  in  the  sizing.  Both  these 
substances  are  comparatively  easily  oxidized,  and 
consequently  it  becomes  plain  that  in  the  coated  paper 
we  have  dichromate,  a  good  oxidizer,  in  contact  with 
excellent  reducing  agents.  So  when  light  energy  was 
brought  to  bear  upon  this  state  of  things,  the  organic 
matter  was  oxidized  and  the  dichromate  reduced.  With- 
out the  organic  substances  there  would  have  been  no 
reduction  of  dichromate,  and  so  the  paper  and  sizing 
act  as  sensitizers.  There  is  reason  to  think  that  the 
chemical  reaction  which  takes  place  under  the  influence 
of  light  may  be  expressed  thus : 

Organic  matter  +  Potassium  dichromate  -f  Light 

=  Chromic  oxide  +  Potassium  hydroxide 
+  Organic  matter  (partly  oxidized) 


LIGHT  AND  CHEMICAL  REACTION      37 

Another  reaction  with  dichromate  has  interesting 
applications  in  printing  processes  to  be  discussed  later. 

Experiment  10.  Make  up  a  solution  of  about  0.5 
gram  of  potassium  dichromate  in  5  cubic  centimeters  of 
water.  Soak  in  a  very  little  cold  water  0.5  gram  of 
gelatine ;  when  it  has  swelled,  add  more  water  so  that  the 
gelatine  may  dissolve  by  gentle  heating.  Take  about  5 
cubic  centimeters  of  the  gelatine  solution,  add  to  it  a 
cubic  centimeter  of  dichromate  solution,  mix  thoroughly, 
and  coat  some  of  the  mixture  on  a  piece  of  sized  paper. 
Expose  the  dried  paper  for  two  or  three  minutes  under 
a  contrasty  negative,  and  observe  the  effect.  Wash  the 
paper  in  warm  water  and  observe  that  all  the  gelatine 
cannot  be  washed  off  the  paper. 

Gelatine  is  an  organic  substance,  akin  to  the  proteins 
which  are  essential  constituents  of  food-stuffs.  It  is 
made  from  the  bones  and  cartilages  of  animals.  It  is 
soluble  in  cold  water,  ordinarily,  and  its  solubility  is 
much  increased  by  gentle  heat;  but  by  long  continued 
boiling  it  tends  to  become  more  insoluble.  A  solution 
of  gelatine  differs  in  a  marked  way  from  one  of  such  a 
substance  as  sodium  chloride  or  silver  nitrate.  When 
solutions  of  these  salts  are  allowed  to  stand,  the  solid 
material  is,  by  the  gradual  evaporation  of  the  water, 
caused  to  deposit  in  the  form  of  crystals.  Also  if  a 
partition  of  parchment  is  placed  in  a  dish  so  that  a 
salt  solution  can  be  poured  in  one  side  and  pure  water 
in  the  other,  the  parchment  serving  to  keep  the  two 
liquids  separate,  after  a  little  while  it  will  be  found  that 
the  salt  has  passed  through  the  parchment  into  the 
water.  This  process  is  known  as  diffusion,  and  all 
substances  that  form  crystals  have  the  power  of 
diffusing  through  such  a  membrane.  Crystallizing  sub- 


38   CHEMISTRY  FOR  PHOTOGRAPHERS 

stances  are  often  called  crystalloids.  But  if  these  ex- 
periments are  repeated,  using  a  solution  of  gelatine  in- 
stead of  salt,  in  the  first  place  no  crystals  will  be  secured 
under  any  circumstances,  but  rather  the  gelatine 
solution  will  "set,"  or,  as  the  housewife  calls  it,  "jelly." 
In  the  second  place,  none  of  the  gelatine  will  be  able  to 
pass  through  the  membrane ;  it  cannot  diffuse  as  a  salt 
is  able  to  do.  As  distinguished  from  the  crystalloids, 
gelatine  belongs  to  the  class  of  substances  called  "col- 
loids." Gum  arabic  is  another  example  of  a  colloid,  and 
one  that  also  has  photographic  uses. 

Referring  now  to  the  experiment  just  performed,  we 
learned  that  light  possesses  the  power  so  to  affect  a 
mixture  of  gelatine  and  potassium  dichromate  that  the 
gelatine,  previously  quite  readily  soluble  in  water, 
becomes  insoluble.  One  circumstance  in  the  way  this 
reaction  was  conducted  is  worthy  of  special  mention. 
Since  the  gelatine-dichromate  mixture  was  spread  as  a 
coating  upon  the  surface  of  paper,  it  is  evident  that 
wherever  the  light  was  transmitted  by  the  negative  it, 
so  to  speak,  fell  first  upon  the  outer  surface  of  the  coat- 
ing. Then,  as  it  passed  into  the  gelatine  film,  it  was 
more  and  more  absorbed.  Consequently  its  energy  was 
gradually  diminished  in  proportion  to  the  depth  to 
which  it  penetrated.  Therefore  the  maximum  effect  of 
this  light  reaction  occurs  at  the  outside  surface  of  the 
film,  the  greatest  degree  of  insolubility  being  produced 
there;  and  the  insolubility  diminishes  as  the  surface 
of  the  paper  is  approached.  So  we  may  have  as  the 
result  of  the  exposure  of  such  a  film  to  light  an  outside 
layer  of  insolubilized  gelatine  protecting,  as  it  were, 
layers  of  much  greater  solubility  which  lie  beneath  it. 
We  may  as  well  note,  also,  that  the  more  soluble  layers 


LIGHT  AND  CHEMICAL  REACTION      39 

are  protected  upon  the  other  side  by  the  paper.  The 
bearing  of  these  observations  we  shall  have  occasion  to 
take  up  in  a  later  chapter. 

The  photo-chemical  reaction  which  historically  was 
the  first  to  be  discovered,  we  have  reserved  for  the 
conclusion  of  our  theoretical  discussion.  It  was  in  the 
sixteenth  century  that  the  alchemists  noted  the  change 
produced  in  silver  chloride  by  the  action  of  light,  al- 
though they  were  unable  to  form  any  adequate  idea 
of  the  nature  of  this  change.  Not  until  the  last  quarter 
of  the  eighteenth  century,  by  the  Swedish  chemist 
Scheele,  was'  it  shown  that  silver  chloride  when  ex- 
posed to  light  under  water  loses  chlorine.  In  his  ex- 
periments Scheele  found  that  after  light  had  produced 
its  effect  upon  the  compound,  the  water  contained 
chlorine  in  solution,  which  would  form  with  silver 
nitrate  a  precipitate  of  silver  chloride  again.  By  treal;- 
ing  with  ammonium  hydroxide  the  residue  of  darkened 
silver  chloride,  Scheele  also  got  what  he  supposed  to  be 
metallic  silver.  So  he  concluded  that  light  had  reduced 
the  silver  chloride  to  elementary  silver,  gaseous  chlorine 
escaping  in  the  process.  Perhaps  it  will  be  of  interest 
to  repeat  this  experiment  of  Scheele's  made  in  the  year 

1777- 

Experiment  1 1 .  Prepare  a  two  per  cent,  solution  of 
silver  nitrate  by  dissolving  0.2  gram  of  the  solid  in  10 
cubic  centimeters  of  water.  Warm,  and  then  com- 
pletely precipitate  the  silver  from  this  solution  as  silver 
chloride  by  adding  hydrochloric  acid  until  no  further 
change  is  observed.  There  should  now  be  present  a 
slight  excess  of  hydrochloric  acid.  Wash  the  precipitate 
thoroughly  by  decantation,  keeping  it  away  from  strong 
light  as  much  as  possibL.  Prove,  by  testing  as  in  Ex- 


40  CHEMISTRY  FOR  PHOTOGRAPHERS 

periment  3,  that  all  soluble  chloride  has  been  washed 
from  the  precipitate.  If  kept  away  from  the  light,  the 
silver  chloride  will  still  be  white.  Treat  a  small  portion 
of  it  in  another  test  tube  with  a  little  dilute  ammonium 
hydroxide,  and  observe  that  the  solid  is  wholly  dissolved. 
Silver  chloride  is  very  soluble  in  ammonium  hydroxide. 
Cover  the  remainder  of  the  precipitate  with  3  or  4 
cubic  centimeters  of  chloride-free  water  and  expose  for 
some  little  time  to  strong  sunlight.  After  the  exposure, 
pour  off  the  water  into  another  clean  test  tube.  Add  a 
drop  of  silver  nitrate  solution  to  this  water,  and  look 
for  the  formation  of  white  silver  chloride.  If  any  is 
formed,  the  chlorine  must,  if  the  work  has  been  care- 
fully done,  have  been  separated  by  the  light  from  the 
original  silver  chloride.  Cover  again  with  a  little  water 
the  darkened  residue  and  treat  it  with  ammonium 
hydroxide  until  there  is  no  further  solvent  action.  The 
dark  powder  which  is  left  is  what  Scheele  took  for 
elementary  silver.  It  is  not  the  element,  however,  for 
it  is  known  to  contain  chlorine.  It  is  now  generally 
considered  to  be  either  a  sub-chloride  of  silver  or  a  solid 
solution  of  silver  in  silver  chloride.  Whichever  may  be 
the  fact,  it  will  for  our  purposes  be  enough  to  call  it 
silver  sub-chloride,  since  practically  it  is  silver  chloride 
which  has  been  robbed  by  the  light  of  some  of  its 
chlorine.  Thus,  when  light  is  allowed  to  act  upon  silver 
chloride,  the  compound  is,  as  it  were,  given  such  a 
shaking  as  to  set  free  part  of  its  chlorine.  The  shorter 
wave-lengths  are  the  most  effective,  and  particularly 
those  in  the  region  of  the  spectrum  known  as  the  ultra- 
violet. Photographically  speaking,  these  are  the  actinic 
rays.  The  reaction  may  be  expressed : 

Silver  chloride  +  Light  =  Silver  sub-chloride  +  Chlorine 


LIGHT  AND  CHEMICAL  REACTION      41 

Experiment  12.  Prepare  silver  bromide  in  an  exactly 
similar  manner  as  the  chloride  was  made  in  Experi- 
ment 1 1 ,  using  potassium  bromide  as  the  precipitant. 
Try  the  solvent  action  of  ammonium  hydroxide  upon 
a  small  portion  of  the  precipitate  and  note  that  silver 
bromide  dissolves,  but  not  so  readily  as  did  the  chloride. 
Expose  some  of  the  salt  to  light  and  observe  the  darken- 
ing. Reaction: 

Silver  bromide  +  Light 

=  Silver  sub-bromide  +  Bromine 

Experiment  1 3 .  Make  another  experiment  similar  to 
the  preceding,  except  that  silver  iodide  is  precipitated, 
washed,  and  examined,  using  potassium  iodide  in- 
stead of  the  bromide.  Compare  the  solubility  of  silver 
iodide  in  ammonium  hydroxide  with  that  of  silver 
bromide  and  silver  chloride  in  the  same  reagent.  Silver 
iodide  is  practically  insoluble  in  the  ammonia.  Ob- 
serve that  upon  very  carefully  washed  silver  iodide 
light  produces  little  effect.  Add  a  drop  of  silver  nitrate 
solution  and  note  that  darkening  proceeds  rapidly. 

Silver  nitrate  is  therefore  a  sensitizer  for  the  iodide. 
That  it  acts  in  the  same  capacity  with  the  chloride  and 
bromide  may  be  quickly  determined  by  experiment. 
The  explanation  of  this  action  on  the  part  of  silver 
nitrate  is  as  follows :  All  three  of  these  photo-chemical 
reactions  are  essentially  reversible,  i.e.,  for  example, 
silver  sub-bromide  and  liberated  bromine  tend  to  re- 
combine  as  fast  as  they  are  formed.  In  the  cases  of 
chloride  and  bromide,  light  energy  is  powerful  enough 
to  cause  decomposition  unaided,  although  it  proceeds 
with  much  greater  rapidity  in  the  presence  of  silver 
nitrate  or  any  other  substance  which  will  take  up 
chlorine  or  bromine.  With  the  iodide,  on  the  other 


42   CHEMISTRY  FOR  PHOTOGRAPHERS 

hand,  there  must  be  present  some  "sensitizer"  in  order 
that  the  reaction  may  go  on.  Reaction : 

Silver  iodide  (with  sensitizer)  +  Light 

=  Silver  sub-iodide  +  Iodine 

In  the  types  of  chemical  reactions  described  in 
Chapter  II,  together  with  the  photo-chemical  ex- 
periments performed  and  discussed  in  the  foregoing 
chapter,  we  have  the  basis  of  all  the  operations  of  the 
photographic  art.  It  is  to  the  application  of  the  princi- 
ples here  so  briefly  set  forth  that  we  now  turn  in  the 
succeeding  chapters. 


CHAPTER  IV 
APPLIED  PHOTO-CHEMISTRY  OF  SILVER  SALTS 


A  COMPARISON  of  the  action  of  light  upon  the 
three  silver  salts,  the  chloride,  bromide,  and  iodide, 
shows  that  the  visible  effect  is  greatest  with  chloride 
and  least  with  iodide.  But  since,  as  we  know,  it  is  not 
metallic  silver  which  is  the  product  of  this  reaction, 
but  only  the  relatively  unstable  sub-compounds,  there 
must  be  provided  some  means  whereby  the  partial  re- 
duction initiated  in  this  photo-chemical  process  may 
be  carried  to  a  finish  in  completely  reduced  silver.  A 
little  experiment  upon  the  interaction  of  reducing 
agents  and  silver  salts  will  serve  to  throw  considerable 
light  upon  this  phase  of  the  problem. 

Experiment  14.  To  a  few  cubic  centimeters  of  silver 
nitrate  solution,  add  a  little  ferrous  sulphate  in  solution 
and  observe  the  immediate  separation  of  silver  as  a 
black  powder.  Ferrous  sulphate  is  a  powerful  reducing 
agent.  Next  acidify  a  little  ferrous  sulphate  solution 
by  adding  a  drop  or  two  of  dilute  sulphuric  acid,  and 
treat  with  this  another  portion  of  silver  nitrate  solu- 
tion. The  reduction  of  silver  takes  place  very  much 
more  slowly  when  acid  is  present.  Next  prepare  small 
quantities  of  thoroughly  washed  silver  chloride,  silver 
bromide,  and  silver  iodide,  taking  care  to  keep  them 
away  from  the  light  as  much  as  possible.  Treat  a 
portion  of  each  salt  with  acidified  ferrous  sulphate. 
There  is  little  reduction  when  the  silver  salt  has  not 
been  subjected  to  the  action  of  light.  Spread  out  upon 
glass  plates  more  of  the  three  salts  and  expose  them  for 

43 


44   CHEMISTRY  FOR  PHOTOGRAPHERS 

five  or  ten  minutes  to  strong  sunlight.  Then  put  a  few 
drops  of  the  acidified  ferrous  sulphate  upon  each  salt 
and  note  that  reduction  at  once  takes  place.  Finally 
expose  to  light  for  a  very  brief  interval,  not  over  half  a 
minute,  the  rest  of  the  silver  salts  and  treat  with  acidi- 
fied ferrous  sulphate.  In  this  case  it  is  to  be  noted  that, 
whereas  the  chloride  is  the  most  visibly  affected  by 
light  and  the  iodide  is  the  least  affected,  reduction 
by  ferrous  sulphate  of  the  exposed  substances  is  most 
marked  with  the  iodide.  Reactions : 

(1)  Silver  nitrate  +  Ferrous  sulphate 

=  Silver  +  Ferric  sulphate  +  Ferric  nitrate 

(2)  Silver  bromide  +  Light 

=  Silver  sub-bromide  +  Bromine 

(3)  Silver  sub-bromide  -f-  Ferrous  sulphate 

=  Silver  +  Ferric  sulphate  +  Ferric  bromide 

From  the  experiment  it  might  naturally  be  inferred 
that  silver  iodide  would  be  the  most  sensitive  medium 
witji  which  to  make  a  coating.  In  practice  a  mixture 
of  bromide  and  iodide  gives  the  best  results,  a  very 
much  greater  proportion  of  bromide  than  of  iodide 
being  used.  In  the  past,  various  different  methods  have 
been  employed  for  supporting  the  light-sensitive  film, 
and  for  fastening  it  to  its  support.  Since  we  are  not  here 
concerned  with  any  of  these  older  methods,  largely  ob- 
solete at  the  present  time,  the  reader  is  referred  for  de- 
tailed accounts  of  them  to  those  works  on  photography 
in  which  is  given  a  historical  survey  of  the  subject.  The 
progress  of  the  art  has  consistently  been  in  the  direction 
of  increased  sensitiveness.  Inasmuch  as  the  general  prin- 
ciples underlying  all  the  different  procedures  have  been 
the  same,  we  shall  not  only  save  time  but  arrive  at  a 
satisfactory  understanding  of  the  matter  by  confining 
ourselves  to  the  discussion  of  present-day  practice. 


PHOTO-CHEMISTRY  OF  SILVER         45 

As  supports  for  the  sensitive  body  there  are  now  in 
use  principally  three  different  materials,  glass  plates, 
celluloid  sheets,  either  cut  in  rectangles  or  in  rolls,  and 
a  considerable  variety  of  papers.  These  may  be 
divided  into  two  groups,  the  glass  and  celluloid  on  the 
one  hand  and  the  papers  on  the  other.  The  former  are 
employed  in  the  first  part  of  the  photographic  process, 
namely,  the  making  of  the  negative;  the  latter  chiefly 
in  the  second  part,  the  making  of  the  print.  This 
distinction  may  reasonably  be  made  for  our  conven- 
ience, but  cannot  be  considered  a  hard  and  fast  rule,  for 
paper  negatives  are  often  made,  and  the  use  of  glass  and 
celluloid  for  lantern-slides  and  transparencies  and  of 
celluloid  for  motion-picture  reels  must  be  taken  into 
account.  But  by  this  classification  we  are  enabled  to 
treat  separately  the  principles  involved  in  the  making 
of  negatives,  leaving  the  numerous  printing  processes 
for  later  chapters.  Of  the  relative  advantages  of  glass 
plates  and  celluloid  sheets  as  supporting  media,  it  is 
needless  here  to  say  more  than  a  word.  Glass  is  heavy 
and  celluloid  is  light  when  it  comes  to  carrying  large 
quantities,  and  the  question  is  easily  answered  by  the 
factor  of  weight,  taking  into  account  also  the  con- 
venience of  the  roll  form  in  which  celluloid  film  is 
supplied.  For  the  amateur  who  gets  beyond  the  "snap- 
shot" stage  and  begins  really  to  be  initiated  into  the 
photographic  "mysteries,"  glass  plates  are  almost  cer- 
tain to  be  found  most  desirable,  partly  on  account  of 
their  lower  price,  but  chiefly  because  they  can  be  had 
of  different  qualities  to  suit  different  purposes. 

So  far,  we  have  considered  the  light-sensitive  sub- 
stance and  the  foundation  upon  which  it  is  laid  down. 
Now  it  remains  to  bring  these  two  factors  together. 


46  CHEMISTRY  FOR  PHOTOGRAPHERS 

Whatever  be  the  nature  of  this  foundation,  the  link  that 
binds  them  together  is  of  the  same  general  character. 
Confining  ourselves  to  glass  as  a  basis  for  simplicity's 
sake,  we  find  that  the  ordinary  "dry  plate,"  or  glass 
plate,  consists  of  a  coating  of  gelatine  containing  the  sil- 
ver salt  and  sometimes  other  materials,  spread  upon  one 
surface  of  a  rectangle  of  clear,  uniform  glass.  The  next 
experiment  illustrates  the  nature  of  this  gelatine  coating. 
Experiment  15.  Soak  in  25  cubic  centimeters  of 
cold  water  contained  in  a  small  conical  flask  1.5  gram 
of  gelatine.  As  soon  as  the  gelatine  has  swelled,  dissolve 
it  by  gentle  warming.  Next  dissolve  3.5  grams  of 
silver  nitrate  in  5  cubic  centimeters  of  water  and  mix 
thoroughly  with  the  former  solution.  In  10  cubic 
centimeters  of  water  dissolve  together  o.i  gram  of 
potassium  iodide  and  3.0  grams  of  potassium  bromide. 
Warm  the  gelatine  mixture  and,  by  the  red  light  of  the 
darkroom,  add  to  it  the  solution  of  bromide  and  iodide 
drop  by  drop,  shaking  it  up  thoroughly  after  each 
addition.  By  this  means  there  are  formed  everywhere 
throughout  the  mass  of  gelatine  minute  solid  particles 
consisting  of  silver  bromide  and  silver  iodide.  Such  a 
mixture  of  liquid  with  finely  divided  solid  particles 
suspended  in  it  is  called  an  emulsion.  Set  the  little 
flask  in  a  dish  containing  boiling  hot  water  and  keep  it 
at  that  temperature  for  half  an  hour,  protecting  it 
from  all  white  light  during  this  operation.  Transfer 
the  emulsion  to  a  small  wide-mouthed  bottle  such  as  a 
vaseline  jar,  for  example;  stand  the  jar  in  cold  water, 
and  allow  the  emulsion  to  set  to  a  firm  consistency. 
It  will  now  be  in  order  to  consider  just  what  the 
emulsion  contains.  According  to  the  proportions  given 
for  silver  nitrate  and  potassium  bromide  and  iodide, 


PHOTO-CHEMISTRY  OF  SILVER         47 

there  is  an  excess  of  the  potassium  salts  present  above 
the  amount  required  to  form  silver  bromide  and  silver 
iodide  with  the  given  quantity  of  silver  nitrate.  Con- 
sequently there  are  now  present  in  the  gelatine  emulsion 
the  unused  potassium  salt  and  the  potassium  nitrate 
formed  in  the  metathesis.  It  is  necessary  to  free  the 
emulsion  from  these  soluble  salts.  As  it  is  very  sensitive 
to  light,  all  these  operations  must  be  performed  in  the 
darkroom.  When  the  emulsion  has  set  thoroughly, 
take  it  from  the  jar  and  put  it  into  the  middle  of  a 
small  square  of  white  mosquito  netting.  Gather  up  the 
corners,  twist. chem  together,  and  squeeze- the  jelly 
through  the  cloth  into  a  dish  of  cold  water,  holding  the 
cloth  under  the  surface,  which  will  prevent  the  shredded 
emulsion  from  running  together  again.  Put  the  shreds 
back  in  the  cloth,  pour  cold  water  liberally  over  them, 
and  Jet  them  soak  in  cold  water  for  a  quarter  of  an 
hour.  Repeat  the  process  of  squeezing  through  the 
cloth  and  washing,  after  which  allow  the  water  to  drain 
off,  and  then  by  gentle  heat  melt  up  the  emulsion  and 
pour  a  portion  of  it  upon  a  small,  clean,  dust- free  glass 
plate,  tilting  the  plate  in  such  a  way  as  to  allow  the 
emulsion  to  run  completely  over  its  surface.  Any  ex- 
cess of  gelatine  may  be  run  from  one  corner  back  into 
the  jar.  Put  the  plate  down  level  to  permit  the  emulsion 
to  set  and  dry.  When  thoroughly  dried  this  "dry  plate" 
can  be  exposed  under  a  negative,  developed,  fixed,  and 
washed  in  the  usual  way.  Coat  several  more  small 
squares  of  glass  with  the  rest  of  the  emulsion  which  has 
been  prepared ;  when  dry  they  may  be  stored  in  a  light- 
proof  box  for  further  experiments. 

All  the  reactions  which  take  place  in  the  operations 
of  making  and  exposing  this  plate  have  already  been 


48   CHEMISTRY  FOR  PHOTOGRAPHERS 

described.  The  reason  for  using  an  excess  of  bromide 
and  iodide  in  the  manufacture  of  photographic  emul- 
sions is  that  silver  nitrate  and  gelatine  interact  chemi- 
cally. The  compound  formed  by  their  reaction  will  also 
react  with  the  bromide  and  iodide  salts  to  form  silver 
bromide  and  silver  iodide.  But,  were  there  an  excess  of 
silver  nitrate  used,  this  excess  could  not  be  washed  out, 
since  the  silver  would  be  chemically  combined  with 
the  gelatine.  On  the  other  hand,  when  the  excess  of 
bromide  and  iodide  is  added,  all  the  silver  present  in 
the  gelatine  solution  reacts  by  metathesis  with  them. 
Then  the  excess  of  soluble  potassium  salts  can  readily 
be  removed  by  washing.  If  the  solutions  out  of  which 
the  emulsion  is  made  are  comparatively  cold  when 
mixed  together,  suitable  examination  will  show  that 
the  particles  of  insoluble  silver  salt  are  very  minute. 
A  plate  coated  with  such  an  emulsion  will  be  relatively 
"slow,"  i.e.,  it  will  require  a  long  exposure  to  light  to 
get  a  satisfactory  image.  In  the  manufacture  of  dry 
plates  it  was  early  discovered,  first,  that  they  could  be 
made  more  rapid  by  allowing  the  emulsion  to  stand  at  a 
temperature  of  about  30  degrees  Centigrade  for  some 
days,  a  process  called  ripening;  and  second,  that  a 
similar  ripening  could  be  secured  by  keeping  the 
emulsion  at  the  boiling  temperature  for  only  a  few 
minutes;  and  finally,  that  simply  the  addition  of 
ammonium  hydroxide  produced  the  same  result  with- 
out any  heating  at  all.  The  reason  for  the  physical 
difference  between  the  unripened  and  the  ripened 
emulsion  can  very  easily  be  shown  by  a  simple  ex- 
periment. 

Experiment  16.  Treat  a  little  cold  dilute  solution  of 
silver  nitrate  with  a  few  drops  of  dilute  hydrochloric 


PHOTO-CHEMISTRY  OF  SILVER         4Q 

acid  without  shaking.  The  precipitated  silver  chloride 
is  seen  to  be  in  a  very  finely  divided  state.  Now  add 
more  of  the  acid  until  precipitation  is  complete  and 
warm  the  mixture  to  boiling.  Shake  it  vigorously. 

The  precipitate  now  is  gathered  in  larger  bunches, 
or,  as  the  chemist  says,  is  flocculent.  So  silver  chloride 
may  be  formed  in  a  very  fine  state  of  division,  but  the 
minute  particles,  when  heated  and  shaken  tend  to 
collect  together  into  larger  aggregations.  The  same 
thing  is  true  of  silver  bromide  and  silver  iodide.  In 
general,  the  more  rapid  the  emulsion  the  larger  the 
particles  of  silver  salt  in  it,  or,  as  the  photographer 
says,  the  coarser  the  grain  of  the  plate.  Thus  the  slower 
plates  have  the  advantage  over  fast  emulsions  of  fine- 
ness of  grain,  which  is  a  point  that  it  is  sometimes 
useful  to  remember.  It  is  not  now  considered  that  the 
rapidity  of  an  emulsion  is  dependent  upon  the  size  of 
its  particles,  but  that  the  increase  in  size  is  an  accessory 
circumstance  to  the  ripening. 

We  may  now  review  briefly  the  reactions  which  take 
place  when  one  of  our  home-made  dry  plates  is  sub- 
mitted to  the  action  of  light  for  a  short  interval  of 
time.  Examination  after  the  exposure  discloses  no 
perceptible  change  in  its  appearance,  but  from  our 
previous  experiments  we  may  be  sure  that,  wherever 
the  light  has  fallen  upon  the  sensitive  emulsion,  silver 
bromide  has  been  altered  to  silver  sub-bromide  and 
silver  iodide,  to  sub-iodide.  In  the  case  of  the  iodide 
we  have  learned  that  the  condition  necessary  for  this 
action  is  that  there  should  be  present  some  substance 
which  is  capable  of  reaction  with  the  liberated  iodine. 
And  with  the  bromide  also  the  action  is  very  much 
facilitated  by  the  presence  of  a  bromine-absorbent, 


50      CHEMISTRY  FOR  PHOTOGRAPHERS 

in  order  to  prevent  the  reverse  reaction  from  occurring. 
Gelatine,  as  it  fortunately  happens,  is  such  an  ab- 
sorbing substance.  Thus  when  the  light  energy  is 
received  in  the  emulsion,  it  is  transformed  into  the 
chemical  energy  of  separation  of  bromine  and  iodine 
from  bromide  and  iodide  respectively,  and  the  free 
bromine  and  iodine  immediately  interact  with  the 
gelatine.  In  this  way  they  are  safely  removed  from  the 
sphere  of  chemical  action.  It  is  chiefly  due  to  this  fact 
that  the  present  day  photographic  emulsion  is  so 
extremely  rapid.  That  this  partial  reduction  has  been 
effected  by  the  exposure  to  light  may  finally  be  dem- 
onstrated by  flowing  over  the  plate,  in  the  darkroom,  a 
little  ferrous  sulphate  solution  acidified  with  sulphuric 
acid.  Any  portions  of  the  plate  which  were  in  the  ex- 
posure protected  from  the  light  will  be  unchanged,  but 
wherever  the  light  reached  the  emulsion  an  immediate 
reduction  of  the  sub-salt  to  metallic  silver  will  occur. 

If  in  the  exposure  the  whole  surface  of  the  plate  is 
not  uniformly  illuminated,  but  different  portions  are 
subjected  to  light  of  different  intensities,  in  the  various 
regions  different  amounts  of  sub-salt  are  formed.  This 
follows  from  the  fact  that  the  quantity  of  silver  salt 
reduced  to  sub-salt  is  directly  proportional  to  the 
amount  of  light  absorption.  But  since  the  whole  of  the 
plate  was  exposed  for  the  same  length  of  time,  the 
quantity  of  light  absorption  is  directly  proportional 
to  the  intensity.  So,  when  the  plate  is  developed  by 
ferrous  sulphate,  in  those  portions  affected  by  more 
intense  illumination  there  is  a  proportionately  greater 
amount  of  silver  reduced  than  in  the  parts  affected 
by  less  illumination.  At  the  upper  limit,  or  maximum 
effect,  all  the  silver  in  the  part  of  the  emulsion  con- 


PHOTO-CHEMISTRY  OF  SILVER         51 

cerned  will  be  reduced  to  metallic  silver.  In  the  mini- 
mum, no  reduction  whatever  will  take  place.  And 
between  these  two  limits  there  is  possible  theoretically 
an  infinite  number  of  steps.  We  have  here  an  ex- 
planation of  the  exquisite  gradation  of  tones  which 
forms  one  of  the  pre-eminent  characteristics  of  the 
photographic  print.  Practically  the  number  of  grades  is 
limited  on  account  of  the  fact  that  the  particles  of 
silver  salt  have  appreciable  size,  and  therefore  the 
number  of  particles  in  a  given  area  varies  with  the  size 
of  the  individual  particles.  Thus  the  depth  of  a  given 
tone  depends'  upon  the  number  of  particles  of  silver 
salt  reduced  to  silver,  and  the  least  possible  difference 
in  tone  is  that  due  to  a  difference  of  one  in  the  number 
of  particles.  Therefore  a  coarse-grained  emulsion,  or 
other  things  being  equal,  a  rapid  one,  necessarily  gives 
less  gradation  than  a  fine-grained,  or  slow,  emulsion. 

So  far  we  have  considered  only  the  effect  of  white 
light.  But,  as  we  know,  white  light  consists  of  a 
mixture  of  an  infinite  number  of  different  wave-lengths 
in  the  luminiferous  ether,  and  we  have  previously 
stated  that  some  photo-chemical  reactions  are  especially 
promoted  by  particular  sets  of  wave-lengths,  such  as, 
for  example,  the  change  of  yellow  phosphorus  to  its  red 
modification,  in  which  the  blue,  violet,  and  ultra-violet 
rays  are  effective.  If  we  should  expose  some  of  our 
home-made  dry  plates  to  the  different  sets  of  wave- 
lengths, or  colored  rays,  of  the  spectrum,  which  is  pro- 
duced by  passing  a  beam  of  white  light  through  a 
prism,  a  very  long  exposure  indeed  would  be  necessary 
to  produce  upon  development  a  perceptible  effect  with 
the  infra-red  rays,  which  are  invisible  of  course  because 
their  wave-lengths  are  too  great.  With  the  red  rays 


52   CHEMISTRY  FOR  PHOTOGRAPHERS 

hardly  less  exposure  would  be  needed  to  give  an 
appreciable  image.  The  yellow  would  require  some- 
what less.  But  even  in  the  green  the  exposure  would  yet 
have  to  be  markedly  long,  although  very  much  shorter 
than  with  the  red  rays.  When  we  came  to  the  blue  and 
violet,  however,  we  should  find  the  sensitiveness  of 
the  plate  as  notable  as  was  its  lack  of  sensitiveness  at 
the  other  end  of  the  spectrum.  And  in  the  ultra-violet, 
invisible  because  of  the  shortness  of  wave-length,  the 
sensitiveness  is  but  a  little  diminished.  To  put  the 
matter  in  another  way,  if  a  strip  of  dry  plate  is  ex- 
posed to  the  whole  spectrum  at  once,  little  or  no  effect 
is  produced  by  the  infra-red  and  red  rays,  and  very 
little  by  the  orange  and  yellow  even  with  a  long  ex- 
posure. A  more  pronounced  effect  begins  in  the  green, 
the  maximum  occurs  in  the  blue-violet,  and  there  is  a 
falling  off  in  the  ultra-violet.  The  sensitiveness  of  the 
eye  to  the  various  portions  of  the  spectrum  is  quite 
different  from  this.  The  infra-red,  it  is  true,  makes  no 
impression  upon  the  sense  of  sight,  but  beginning  with 
the  red  its  sensitiveness  rapidly  increases  to  a  maximum 
in  the  yellow.  From  here  the  brightness  diminishes 
again  until  it  reaches  zero  once  more  in  the  ultra-violet. 
To  the  eye  the  brightest  part  of  the  spectrum  is  the 
region  from  orange  through  yellow  to  green,  parts  that 
are  practically  dark  to  ordinary  photographic  emulsion ; 
whereas  to  the  emulsion  'the  brightest  portion  lies  in 
the  blue-violet,  which  is  comparatively  dark  to  the  eye, 
and  the  brightness  extends  beyond  into  the  ultra-violet, 
which  is  totally  black  to  the  eye.  Since  daylight 
ordinarily  contains  a  preponderance  of  blue,  violet,  and 
ultra-violet  rays,  and  red,  orange,  and  yellow  rays  act 
to  no  appreciable  extent  upon  the  ordinary  emulsion,  a 


PHOTO-CHEMISTRY  OF  SILVER          53 

plate  coated  with  it  and  exposed  in  the  camera  to  a 
landscape,  for  example,  registers  the  view  as  it  would 
appear  to  an  eye  whose  sensitiveness  instead  of  having 
its  maximum  in  the  yellow  has  this  maximum  in  the 
blue-violet,  the  same  as  the  plate.  The  landscape,  too, 
commonly  has  an  abundance  of  green  foliage,  to  the 
rays  from  which  the  emulsion  is  but  little  sensitive. 
Using  such  emulsion,  it  is  a  matter  of  no  small  difficulty 
photographically  to  make  a  landscape  which  will  show 
the  verdure  in  foreground,  the  distance,  and  the  sky 
as  these  factors  appear  to  a  discriminating  eye.  To 
represent  the  blue  sky  and  even  the  billowy  clouds  by  a 
practically  uniform  expanse  of  white  paper,  and  the 
varied  greens  of  grass  and  trees  by  dense  blackness  is 
to  give  a  very  untruthful  rendering. 

The  position  of  the  maximum  sensitiveness  of  the 
photographic  emulsion  cannot  be  moved  from  the  blue- 
violet  region,  but  by  incorporating  with  it  certain 
dyes,  it  is  possible  to  produce  secondary  maxima  in 
other  regions  of  the  spectrum.  Thus,  by  treating  one 
of  our  home-made  plates  with  a  solution  of  the  blue 
dye  cyanin,  and  allowing  it  to  dry  again,  we  shall  have 
a  plate  which  is  sensitive  far  into  the  red.  A  purple 
solution  of  pinacyanol  also  gives  red-sensitiveness  and 
great  rapidity  to  the  plate,  but  one  so  prepared  must 
be  handled  and  developed  by  a  green  light,  for  it  is 
quite  insensitive  to  the  green  rays.  With  pinaverdol 
there  is  a  great  increase  in  the  sensitiveness  to  the 
orange,  yellow,  and  green.  If  these  dyes  are  added  in 
the  process  of  manufacturing  the  emulsion,  the  plates 
so  prepared  have  fair  keeping  qualities.  Plates  thus 
made  keep  better,  but  are  not  quite  so  color-sensitive 
as  those  which  are  dyed  just  before  using.  The  action 


54      CHEMISTRY  FOR  PHOTOGRAPHERS 

of  these  dyes  in  producing  red,  yellow,  and  green 
sensitiveness  is  not  wholly  clear.  Possibly  they  form 
very  complex  silver  compounds  which  are  specially 
sensitive  to  the  respective  rays.  Inasmuch  as  it  is  the 
light  absorbed  which  performs  the  chemical  work, 
the  suggestion  has  been  made  also  that  perhaps  the 
dye,  after  absorbing  its  particular  rays,  is  able  to  give 
out  again  the  energy  so  stored  up.  Where  these  rays 
act  upon  the  plate,  therefore,  an  increased  reduction 
of  silver  salt  occurs.  Color-sensitized  plates  are  sold 
under  the  names  isochromatic,  orthochromatic,  pan- 
chromatic, etc.  But  as  already  stated,  the  maximum 
sensitiveness  to  the  violet  end  of  the  spectrum  cannot 
be  changed  by  any  means  that  has  so  far  been  dis- 
covered, and  even  when  red-sensitiveness  has  been 
increased  as  much  as  possible,  the  blue-sensitiveness 
still  overbalances  it.  For  this  reason  it  is  necessary  in 
order  to  secure  the  full  benefit  of  color  correction  to  cut 
out  most  of  the  blue,  violet,  and  ultra-violet  rays  from 
the  light  which  is  transmitted  to  the  plate.  This  is 
done  by  putting  over  the  lens  a  piece  of  yellow  glass, 
called  a  ray  filter  or  ray  screen.  Being,  as  we  say, 
colored  yellow,  this  glass  has  the  property  of  absorbing 
the  rays  from  the  blue  end  of  the  spectrum  and  so 
protects  the  plate  from  their  action.  A  disadvantage 
in  the  use  of  ray  filters  lies  in  the  fact  that  the  artificial 
sensitiveness  of  the  plate  to  the  red  cannot  be  made  as 
great  as  its  natural  sensitiveness  to  the  blue.  With  the 
ray  filter  the  plate  is  therefore  much  less  rapid  than 
without  it. 

Thus  far,  in  speaking  of  the  exposure  of  our  emulsion 
to  light,  we  have  been  concerned  merely  with  that 
duration  of  exposure  which  would  give  an  appreciable 


PHOTO-CHEMISTRY  OF  SILVER          55 

image  by  use  of  such  a  reducing  agent  as  ferrous 
sulphate,  for  example,  or  as  we  should  say,  photo- 
graphically, upon  development.  If  we  now  inquire 
what  will  be  the  effect  of  submitting  a  plate  to  ex- 
posures of  various  durations,  we  shall  obtain  some  very 
interesting  results.  Let  us  suppose  that  with  the  con- 
ditions of  illumination,  the  size  of  stop  it  is  desired  to 
use  in  the  lens,  and  the  sensitiveness  of  the  plate 
employed,  an  exposure  of  one  second  would  give  us 
what  the  photographer  is  wont  to  call  a  "perfectly 
timed"  negative.  We  recognize  at  once  that,  if  we 
allow  but  a 'very  small  fraction  of  a  second  for  the 
exposure,  there  will  not  be  time  enough,  or  much 
better,  not  energy  enough  in  the  light  which  reaches 
the  plate,  to  affect  more  than  a  very  small  proportion 
of  the  sensitive  silver  salt.  Thus,  we  would  get  an 
"under-exposure,"  a  sort  of  negative  generally  char- 
acterized by  a  few  dense  patches  corresponding  to  the 
highest  lights  in  the  object,  and  clear  glass,  giving  none 
of  the  details  corresponding  to  the  deepest  shadows. 
By  making  a  series  of  exposures,  each  one,  say,  double 
the  preceding,  we  find  as  we  progress  towards  the 
standard  exposure  of  one  second  that  finally  a  point  is 
reached  where  a  negative  can  be  developed  which  com- 
pares quite  favorably  with  one  perfectly  timed,  and  yet 
the  duration  of  the  exposure  which  produced  it  was 
considerably  less  than  the  normal.  Furthermore,  by 
carrying  our  series  of  exposures  up  above  one  second, 
we  can  discover  an  upper  limit  at  which  a  passable 
negative  will  also  be  secured.  This  range  of  expos- 
ures within  which  acceptable  negatives  can  be  made 
is  commonly  called  the  "latitude"  of  the  emulsion, 
and  varies  somewhat  with  different  brands  of  plates 


56   CHEMISTRY  FOR  PHOTOGRAPHERS 

and  with  different  speeds  of  emulsion.  It  is  the  safe- 
guard of  the  amateur.  In  general,  slower  plates  have 
greater  latitude  than  the  faster  ones.  But  above  this 
upper  limit,  the  negatives  which  result  become  more 
and  more  disappointing.  They  are  indeed  "flat,  stale, 
and  unprofitable."  The  best  cure  for  overexposure, 
as  it  is  also  for  underexposure,  is  to  throw  away  the 
plates  and  try  again,  using  an  exposure-meter  to 
determine  the  correct  time. 

But  suppose  we  allow  light  to  act  for  periods  succes- 
sively very  much  greater  than  the  normal  one  second. 
Presently  upon  development  one  of  them  will  come  up  a 
positive,  instead  of  a  negative,  highlights  in  the  plate 
now  corresponding  to  highlights  in  the  object,  and 
shadows  to  shadows.  This  being  the  reverse  of  what 
usually  happens,  is  known  as  the  "reversal"  of  the 
image.  A  further  increase  in  the  length  of  exposure 
will  give  a  negative  again;  still  more  time,  a  positive; 
and  yet  more,  a  negative. 

Experiment  17.  Under  such  conditions  as  would 
normally  demand  an  exposure  of  about  one  second, 
submit  an  ordinary  plate  or  film  in  a  camera  to  the 
action  of  light  from  some  object  for  about  one  minute, 
and  develop  as  usual.  Give  to  another  plate  an  ap- 
proximately normal  exposure;  in  the  darkroom  place 
it  for  two  minutes  in  a  dilute  solution  of  potassium 
dichromate,  rinse  off  the  dichromate,  and  then  try  to 
develop  it  into  a  negative.  Consider  that  the  silver 
salt  which  has  been  partially  reduced  by  light  has  first 
been  subjected  to  the  action  of  a  good  oxidizer  (di- 
chromate) before  the  application  of  the  developer. 
Give  to  a  third  plate  an  exposure  approximately  normal. 
This  will  naturally  develop  into  a  negative.  When  it 


PHOTO-CHEMISTRY  OF  SILVER          57 

has  been  developed,  rinse,  and  put  it  into  a  dilute 
solution  of  dichromate,  and  carry  it  out,  in  the  tray, 
into  daylight.  As  soon  as  the  plate  has  bleached  out, 
wash  it  in  running  water  for  half  a  minute  and  develop 
it  again,  in  daylight,  with  the  same  developer  as  before 
used.  The  image  should  be  reversed. 

Any  other  soluble  oxidizing  agent,  such  as  potassium 
permanganate,  hydrogen  dioxide,  etc.,  would  do  just 
as  well  as  the  dichromate.  In  explaining  this  curious 
behavior  of  the  light-sensitive  emulsion,  we  must  take 
several  things  into  account.  The  silver  sub-salt  formed 
by  the  reducing  action  of  light  is  not  only  readily 
capable  of  being  further  reduced  by  suitable  reducing 
agents,  but,  since  the  reaction  is  essentially  a  reversible 
one,  in  the  absence  of  a  sensitizer  (gelatine  in  this  case) , 
the  sub-salt  is  quite  as  readily  oxidized  again.  Thus, 
silver  bromide,  sealed  up  in  a  glass  tube  from  which 
the  air  has  been  exhausted,  when  submitted  to  light  is 
separated  into  bromine  and  silver  sub-bromide.  But  as 
soon  as  the  tube  is  taken  away  from  the  light,  bromine 
and  sub-bromide  commence  to  recombine,  forming  sil- 
ver bromide  again.  By  experiment  with  the  different 
regions  of  the  spectrum,  it  has  been  shown  that  the 
reversing  effect  is  produced  by  the  rays  of  greater 
wave-length  (i.e.  from  the  lower,  or  red,  end  of  the 
spectrum).  Taking  this  fact  in  connection  with  our 
experiment  in  which  potassium  dichromate  prevented 
the  negative  image  from  being  formed  after  a  normal 
exposure,  we  see  that  when  a  plate  is  exposed  for  a 
sufficient  length  of  time,  the  silver  sub-salt  formed,  so 
to  speak,  by  the  short  wave-lengths  in  the  normal 
part  of  this  exposure  is  by  the  action  of  the  longer 
wave-lengths  in  the  balance  of  the  exposure  time 


58   CHEMISTRY  FOR  PHOTOGRAPHERS 

caused  to  unite  with  oxygen  from  the  air,  that  is,  it  is 
reoxidized.  But  in  the  meantime  those  portions  of  the 
silver  salt  which,  after  normal  exposure,  would  have 
remained  unreduced  to  sub-salt,  by  the  excessive  ex- 
posure have  been  so  reduced.  The  condition  of  the 
plate,  consequently,  now  is  such  that  the  portions  of 
silver  salt  which  normally  would  be  reducible  by  the 
developer,  having  been  reoxidized,  are  incapable  of 
reduction;  whereas  those  portions  which  normally 
would  not  have  been  reducible  are  present  as  light- 
reduced  sub-salts  and  therefore  are  reduced  to  silver 
by  the  developer.  So  a  reversed  image  must  be  formed. 
By  incorporating  in  the  emulsion  a  suitable  reducing 
agent,  which  not  only  prevents  the  reoxidation  of  sub- 
salt,  but  also  serves  as  absorbent  of  the  greater  amounts 
of  bromine  set  free  in  prolonged  exposures,  reversal 
can  be  entirely  prevented.  Plates  thus  prepared  are 
incapable  of  forming  reversed  images  even  with  ex- 
posures many  hundred  times  the  normal,  and  thus 
cannot  be  overexposed.  By  this  means  sufficient  ex- 
posure can  be  given  to  secure  details  in  the  shadows 
without  at  the  same  time  completely  blocking  up  the 
highlights.  Hydrazine  salts  are  the  compounds  em- 
ployed for  the  purpose,  and  the  plates  are  called 
Hydrazine,  or  Hydra,  plates.  At  first  thought  it  might 
seem  that  here  is  a  solution  of  the  whole  problem  of 
exposure.  Use  Hydrazine  plates  and  give  plenty  of  time, 
thus  avoiding  the  Scylla  of  underexposure,  there  being 
no  Charybdis.  But  the  matter  is  not  quite  as  simple  as 
this.  There  is  such  a  thing  as  getting  too  much  detail 
even  in  the  shadows.  As  in  all  over-exposed  plates, 
there  is  a  tendency  toward  flatness  in  the  Hydrazine 
plates,  so  that  they  must  be  used  with  discrimination. 


PHOTO-CHEMISTRY  OF  SILVER         59 

There  is  one  defect  inherent  in  glass  plates  which  we 
must  mention  before  concluding  this  chapter.  It  is 
present  very  little  in  celluloid  films,  and  this  fact 
forms  another  point  of  advantage  of  films  over  plates. 
When  an  interior  is  to  be  photographed  in  which 
brightly  illuminated  windows  are  included  in  the  view, 
the  glass  negative  always  shows  much  blurring  about 
these  highlights.  There  is  so  much  reduced  silver  all 
around  the  images  of  the  windows  as  sometimes  to 
obscure  their  outlines.  The  same  effect  is  also  often 
produced  in  out-of-door  work  when  patches  of  sky 
show  through  branches  of  foliage;  and  it  can  even  be 
found  in  portraiture  where  the  subject  is  dressed  in 
white.  The  effect  is  known  as  "halation,"  and  has  a 
very  simple  cause,  and  an  equally  simple  cure.  In  the 
thin  coating  of  emulsion  upon  the  glass  plate  it  is 
impossible  that  all  of  the  impinging  light  should  be 
absorbed.  The  part  that  is  absorbed  by  this  film  per- 
forms, as  we  have  seen,  its  chemical  work  and  pro- 
duces the  latent  photographic  image.  That  portion  of 
the  light  which,  failing  to  be  absorbed,  is  transmitted 
through  the  coating  passes  into  the  glass  behind  it. 
Here  again  a  small  part  is  absorbed  by  the  glass,  while 
the  remainder  finally  reaches  the  under  surface  of  the 
plate.  At  this  surface  another  division  occurs,  part  of 
the  light  passing  through  and  into  the  space  behind  the 
plate,  and  the  rest  suffering  reflection.  It  is  the  re- 
flected part  of  the  light,  which  has  come  through  the 
sensitive  coating  and  the  glass,  which  produces  the 
untoward  effect  of  halation,  for  it  now  is  directed  back 
toward  the  under  side  of  the  gelatine  film,  at  an  angle 
such  as  to  strike  the  film  at  a  different  place  from  where 
it  came  through.  Passing  into  the  emulsion  it,  of  course, 


60   CHEMISTRY  FOR  PHOTOGRAPHERS 

forms  sub-salt  of  silver  all  around  the  latent  image  of 
the  highlight  in  question.  To  prevent  this  reflection  at 
the  back  surface  of  the  glass  is  to  do  away  with  hala- 
tion, and  there  are  two  methods  in  use.  One  is  to  apply 
a  coat  of  slow  emulsion  upon  the  glass,  over  which  a 
second  rapid  film  is  spread.  Such  double-coated  plates 
are  a  great  improvement  over  the  single-coated  variety 
in  several  ways,  but  while  halation  is  very  much 
diminished  by  this  means,  it  is  not  in  fact  entirely 
eliminated.  The  second  method  is  to  coat  the  back 
of  the  plate  with  a  substance  which  will  absorb  all  of 
the  light  which  reaches  that  surface  of  the  glass.  None 
can  then  be  reflected  to  the  under  side  of  the  emulsion. 
Such  a  "backed"  plate  can  be  had  at  a  slight  extra 
cost  over  the  unbacked  kind,  and  once  used  will  prob- 
ably mean  always  used. 


CHAPTER  V 
CHEMISTRY  OF  DEVELOPMENT 


CHEMICALLY  speaking,  the  development  of  a 
negative  consists  in  the  complete  reduction,  to 
metallic  silver,  of  the  light-affected  silver  salt.  In 
order  thus  to  get  a  negative,  it  is  necessary,  of  course, 
that  th&ianaffected  silver  salt  should  escape  such  reduc- 
tion. This  fact,  then,  establishes  the  prime  character- 
istic of  a  photographic  developer,  namely,  that  its 
reducing  power  with  silver  salts  must  be  so  adjusted  as 
to  enable  it  to  act  selectively  upon  these  light-exposed 
compounds  in  the  emulsion,  reducing  the  sub-salts  and 
leaving  unreduced  the  remaining  silver  bromide  and 
silver  iodide.  In  our  experiments  with  the  strong 
reducing  agent,  ferrous  sulphate,  we  learned  that  its 
simple  solution  in  water  is  capable  of  instantly  reducing 
silver  nitrate  to  silver.  But  by  acidifying  the  ferrous 
sulphate,  we  were  able  to  control  and  modify  its  re- 
ducing power,  even  a  few  drops  of  acid  greatly  retarding 
the  reaction,  although  not  preventing  it  entirely  from 
taking  place.  Since  all  chemical  action  involves  energy, 
i.e.,  work,  it  is  plain  that  a  certain  amount  of  chemical 
work  must  be  performed  when  silver  is  reduced  from 
such  a  compound  as  silver  nitrate,  or  silver  bromide. 
Now,  in  consequence  of  the  law  of  conservation  of 
energy,  the  amount  of  work  done  when  a  given  quantity 
of  silver  bromide,  for  example,  is  completely  reduced  to 
silver  is  the  same  as  the  sum  of  the  amounts  con- 
cerned in  the  partial  reduction  of  this  quantity  to  sub- 
bromide  and  the  final  reduction  of  sub-bromide  to 

61 


62   CHEMISTRY  FOR  PHOTOGRAPHERS 

silver.  In  other  words,  it  makes  no  difference  in  the  total 
quantity  of  chemical  work  done  whether  the  reduction 
be  all  at  once  to  silver  or  whether  it  be  carried  out  in  the 
two  steps.  Therefore,  evidently,  since  the  energy  of  light 
has  been  used  in  the  first  step,  that  is,  in  the  reduction 
to  sub-salt  in  the  exposure,  there  is  less  chemical  work 
for  the  reducing  agent  in  the  developer  to  do  in  reduc- 
ing the  sub-bromide  than  in  reducing  the  unaffected  sil- 
ver bromide.  This  explains  why  a  suitably  regulated 
reducing  agent  (the  photographic  developer)  is  able  to 
bring  out  upon  the  plate  in  metallic  silver  the  latent 
image  produced  in  the  exposure  to  light.  Because  less 
chemical  work  is  required  to  be  done  in  the  reduction 
of  silver  sub-bromide  to  silver  than  in  reducing  silver 
bromide  to  the  metal,  the  developer  proceeds  to  act  first 
upon  the  former.  That  it  will  in  time  act  also  upon  the 
silver  bromide  we  shall  later  see. 

Experiment  18.  Make  up  a  0.5%  solution  of  pyro- 
gallic  acid  by  dissolving  o.  i  gram  of  the  solid  in  20 
cubic  centimeters  of  water,  and  mix  by  stirring  with  a 
glass  rod.  Avoid  shaking  the  tube.  Pour  a  few  cubic 
centimeters  into  a  test  tube,  shake  well,  and  note  that 
there  is  no  darkening  of  the  solution.  Add  a  drop  or 
two  of  dilute  sulphuric  acid  to  the  bulk  of  the  solution 
and  mix  by  stirring.  Shake  up  a  little  of  this  solution  in 
a  test  tube.  The  color  change  is  slight.  In  half  of  the 
remaining  liquid  dissolve  0.2  to  0.3  gram  of  dry  sodium 
sulphite,  and  in  the  other  half  about  the  same  quantity 
of  anhydrous  sodium  carbonate.  Shake  up  each  of 
these  mixtures  and  observe  how  rapidly  the  second 
turns  yellow,  whereas  the  first  shows  no  change. 

Pyrogallic  acid  is  an  organic  compound  obtained  by 
heating  gallic  acid,  which  is  a  constituent  of  gall-nuts 


CHEMISTRY  OF  DEVELOPMENT         63 

and  many  other  vegetable  substances  such  as,  for  ex- 
ample, tea.  It  is  a  strong  reducing  agent,  being  itself 
oxidized  in  the  process  to  oxalic  and  acetic  acids.  Thus, 
when  exposed  to  the  air,  its  solutions  absorb  oxygen, 
just  as  do  solutions  of  ferrous  sulphate,  and  oxidation 
of  the  pyrogallic  acid  occurs.  But  this  oxidation  does 
not  take  place  so  rapidly  in  a  plain  solution  of  pyro- 
gallic acid,  or  in  one  containing  free  sulphuric  acid,  as  it 
does  when  the  solution  is  made  alkaline  as  with  sodium 
carbonate.  Also  when  sodium  sulphite,  which  is  like- 
wise a  reducing  agent  and  oxygen  absorber,  is  present 
in  the  solution,  the  oxidation  of  pyrogallic  acid  is 
hindered,  since  the  absorbed  oxygen  is  used  up  first  in 
oxidizing  the  sulphite  to  sulphate. 

Experiment  19.  Dissolve  0.5  gram  of  sodium  car- 
bonate (anhydrous)  in  10  cubic  centimeters  of  water, 
add  to  this  o.  i  gram  of  pyrogallic  acid  and  observe  the 
color  immediately  developing.  Now  add  to  the  mix- 
ture a  few  cubic  centimeters  of  a  strong  solution  of 
sodium  sulphite  and  note  that  the  yellow  color  is  at 
once  bleached,  but  that  after  the  lapse  of  sufficient  time 
discoloration  again  occurs. 

In  alkaline  solution  the  brown  substance  formed  by 
oxidation  is  soluble.  It  is  in  the  nature  of  a  dye,  and  is 
fairly  insoluble  in  water  in  the  absence  of  alkali.  Its 
dilute  solutions  are  yellow,  turning  brown  when  the 
concentration  increases.  Plates  and  films  developed 
with  "pyro"  sometimes  are  stained  a  yellowish  brown 
because  after  the  free  alkali  has  been  washed  out  this 
brown  dye  is  insoluble  in  the  water.  Also,  it  seems  to 
have  a  tendency  to  become  a  "fast"  color  in  the 
gelatine  similar  to  other  dyes  in  fabrics. 

Experiment  20.  Treat  with  a  few  drops  of  sodium 


64   CHEMISTRY  FOR  PHOTOGRAPHERS 

sulphite  solution  a  little  silver  nitrate  dissolved  in  5 
cubic  centimeters  of  water,  and  note  that  white  silver 
sulphite  is  precipitated.  No  reduction  occurs.  Next 
prepare  a  small  quantity  of  washed  silver  bromide 
unexposed  to  white  light.  Dissolve  a  little  of  this  un- 
exposed  salt  in  a  test  tube  with  enough  ammonium 
hydroxide,  add  to  it  a  little  pyrogallic  acid  solution, 
and  observe  the  immediate  blackening  of  the  liquid. 
This  is  due  to  the  reduction  of  silver  and  to  the  dark- 
colored  oxidation  products  of  the  alkaline  pyrogallate. 
Thus  in  alkaline  solution,  pyrogallic  acid  is  able  to  ef- 
fect the  complete  reduction  of  even  unexposed  silver  bro- 
mide. The  essential  condition  for  this  reaction  is  that  the 
silver  bromide  should  be  in  solution.  In  the  operation 
the  pyrogallic  acid  is  oxidized,  and  the  bromine  com- 
bines with  the  alkali  to  form  alkali  bromide,  in  the  case 
mentioned,  ammonium  bromide.  Reactions: 

1 i )  Silver  nitrate  +  Sodium  sulphite 

=  Silver  sulphite  +  Sodium  nitrate 

(2)  Silver  bromide  +  Ammonium  hydroxide 

=  Silver-ammonia  bromide  +  Water 

(3)  Silver-ammonia  bromide  -f-  Pyrogallic  acid 

=  Silver  +  Ammonium  bromide  +  Oxidation 
products  of  pyrogallic  acid 

Experiment  21.  Make  up  a  little  "pyro"  developer, 
in  two  solutions,  as  follows : 

1 .  Sodium  sulphite,  dry 10  grams 

Pyrogallic  acid 3  grams 

Sulphuric  acid  (concentrated) 2  drops 

Water,  to 100  cubic  centimeters 

2.  Sodium  carbonate,  anhydrous 5  grams 

Water,  to 100  cubic  centimeters 

When  ready  to  use  this  developer,  mix  the  two  solu- 
tions and  dilute  to  300  cubic  centimeters  with  water. 
For  carrying  out  this  experiment  take  three  small- 


CHEMISTRY  OF  DEVELOPMENT         65 

sized  dry  plates.  The  first  plate  is  to  be  given  a  normal 
exposure  in  the  camera  or  under  a  negative,  but  should 
be  taken,  like  the  other  two,  from  the  package  in  total 
darkness  when  loading  it  in  the  plateholder.  After  ex- 
posing this  plate,  mix  the  prepared  developer  solutions 
and  put  about  equal  portions  in  each  of  three  trays, 
adding  i  cubic  centimeter  of  a  ten  per  cent,  solution  of 
potassium  bromide  to  the  second  tray.  Turn  off  the 
red  light,  and  in  the  dark  take  out  the  other  two  un- 
exposed  plates  from  the  box  and  the  one  which  has 
been  exposed  from  the  plateholder  and,  as  quickly  as 
possible,  slip  the  three  plates  into  the  trays,  the  un- 
exposed  two  in  the  first  two,  the  exposed  in  the  third 
tray.  Cover  the  trays,  after  which  the  red  light  may  be 
turned  on  again.  At  intervals  for  ten  to  twelve  minutes 
the  covers  may  be  lifted  and  the  plates  examined. 

The  unexposed  plate  in  the  first  tray  will  begin  to 
show  reduction  of  silver,  that  is,  "fog,"  in  about  two 
minutes,  whereas  the  exposed  plate  in  the  third  tray 
will  develop  without  fogging  for  as  much  as  ten  minutes. 
In  the  second  tray  to  which  has  been  added  the  potas- 
sium bromide,  the  unexposed  plate  remains  free  from 
fog  for  at  least  as  long  as  the  exposed  plate.  If  a  fourth 
plate  were  to  be  exposed  normally  and  put  into  a 
solution  containing  pyrogallic  acid,  sodium  sulphite, 
and  acid,  without  the  addition  of  any  of  the  alkali, 
we  should  see  that  development,  otherwise  reduction 
of  silver,  would  be  extremely  slow.  This  effect  might  be 
expected  from  the  discussion  of  Experiment  18,  re- 
calling that  a  solution  of  pyrogallic  acid  containing 
mineral  acid  possesses  far  less  reducing  power  than  one 
containing  free  alkali. 

In  order  to  make  clear  what  is  indicated  by  the  ex- 


66  CHEMISTRY  FOR  PHOTOGRAPHERS 

periment  just  described,  let  us  consider  that  in  the 
gelatine  emulsion  with  which  the  dry  plate  is  coated 
we  have  a  layer  of  gelatine,  of  definite  and  measurable 
thickness,  in  which  are  embedded  minute  lumps, 
spaced  with  some  irregularity,  of  light-sensitive  silver 
salt.  For  convenience  we  will  disregard  the  silver 
iodide  and  consider  only  the  bromide,  since  the  action 
is  essentially  the  same  in  the  case  of  both.  Thus  the 
particles  of  silver  bromide  in  the  body  of  the  gelatine 
may  be  likened  to  plums  in  a  pudding.  The  structure 
of  the  gelatine  itself  may  be  compared  somewhat  to 
that  of  a  honeycomb  if  it  be  imagined  first  that  the 
comb  consists  of  the  same  material  precisely  as  the 
honey,  and  second,  that,  instead  of  the  cells  of  the 
comb  fitting  tightly  together  all  round,  there  are 
spaces  between  in  places,  making  in  effect  tubes  or 
tunnels  into  and  through  the  mass.  It  is  by  means  of 
these  infinitesimal  passageways  that  the  reducing  solu- 
tion which  we  call. the  developer  makes  its  way  into 
the  body  of  the  gelatine  and  thus  comes  into  contact 
with  the  silver  bromide. particles.  If  we  now  limit  the 
discussion  to  a  single  one  of  these  particles  of  bromide, 
we  shall  see  that  its  microscopic  mass  is  surrounded  on 
all  sides  by  comparatively  transparent  gelatine,  but 
that  it  can  be  reached  by  water  and  by  substances  dis- 
solved in  water  through  the  pores  in  the  gelatine.  That 
is,  the  particle  is  accessible  to  both  the  light  and  the 
developer,  but  is  somewhat  protected,  after  all,  from 
the  latter.  Taking  the  first  plate  which  was  put,  un- 
exposed,  into  developer  containing  no  potassium 
bromide,  let  us  see  what  happens  to  our  particle  of 
silver  salt.  First,  let  it  be  noted  that,  microscopic 
though  it  be  in  size,  nevertheless  the  particle  contains 


CHEMISTRY  OF  DEVELOPMENT         67 

a  very  large  number  of  molecules  of  silver  bromide. 
As  soon  as  the  alkaline  pyrogallate  solution  penetrates 
through  the  gelatine  envelope  and  touches  the  surface 
of  the  particle,  silver  bromide  commences  to  dissolve 
in  the  liquid.  That  is,  molecules  of  silver  bromide 
begin  to  pass  from  the  solid  particle  into  the  liquid. 
But  we  have  already  learned  by  an  experiment  that 
alkaline  pyrogallate  reduces  silver  from  a  solution  of 
silver  salt,  and  therefore  it  is  not  surprising  to  find  here 
that  reduction  occurs,  if  we  can  be  assured  that  any 
silver  bromide,  which  we  have  thought  of  as  an  insolu- 
ble compound,  really  goes  into  solution.  As  a  matter 
of  fact,  no  substance  is  really  insoluble  in  the  absolute 
sense.  Silver  bromide,  indeed,  is  soluble  to  the  extent 
of  0.002  gram  in  i  ,000  cubic  centimeters  of  water,  but 
its  solubility  can  be  varied,  as  we  shall  presently  see, 
by  having  other  substances  too  in  the  solution.  So,  as 
the  molecules  of  silver  really  do  go  into  the  liquid  one 
after  another,  and  at  once  are  reduced  there,  the  silver 
from  them  is  deposited  in  solid  form  practically  in  the 
spot  formerly  occupied  by  the  bromide,  the  bromine 
is  taken  up  by  the  liquid,  forming  sodium  bromide  in 
solution,  and  a  corresponding  amount  of  pyrogallic 
acid  is  oxidized.  In  the  experiment  with  the  plate  we 
actually  found  that  silver  was  reduced,  and  we  called 
it  "fogging"  of  the  plate.  This  chemical  "fog,"  there- 
fore, is  simply  the  reduction  of  silver  from  silver  salt 
without  any  mediation  of  light,  and  is  bound  to  take 
place  so  soon  as  silver  bromide  and  alkaline  pyrogallate 
come  together  in  solution. 

But,  as  appeared  from  the  experiment  with  the 
second  unexposed  plate,  such  reduction  can  be  very 
much  hindered.  This  plate  was  put  into  developer 


68   CHEMISTRY  FOR  PHOTOGRAPHERS 

which  differed  from  that  in  the  first  tray  by  containing 
a  small  quantity  of  potassium  bromide.  Since  this  was 
the  only  difference  between  the  two  experiments,  the 
delay  in  the  reduction  of  silver  can  have  been  caused 
only  by  the  presence  of  this  potassium  bromide.  Let 
us  see  how  this  reagent  acts  towards  silver  bromide  in 
solution. 

Experiment  22.  Dissolve  a  little  silver  bromide  with 
a  few  cubic  centimeters  of  dilute  ammonium  hydroxide. 
Add  to  this  solution  2  or  3  cubic  centimeters  of  a  fairly 
strong  solution  of  potassium  bromide  in  water  and 
observe  that  silver  bromide  is  reprecipitated. 

The  solubility  of  silver  bromide  is  therefore  less  when 
potassium  bromide  is  present  in  the  solution  than  in 
the  absence  of  the  latter  salt.  Thus,  when  the  developer 
solution  containing  potassium  bromide  came  in  con- 
tact with  the  gelatine-embedded  silver  bromide  particle 
in  the  second  plate,  on  account  of  the  diminished 
solubility  of  the  silver  bromide  the  molecules  of  this 
compound  were  very  much  hindered  in  their  passing 
into  the  solution  by  the  potassium  bromide  already 
contained  in  it.  Since  the  rate  at  which  the  silver 
bromide  was  going  into  solution  was  in  this  way 
diminished  and  since  also  the  rate  of  its  reduction 
depends  upon  this  rate  of  solution,  it  was  inevitable, 
now  that  we  understand  the  situation,  that  the  re- 
duction, or  otherwise  the  "fogging,"  should  be  retarded 
by  the  addition  of  potassium  bromide.  In  photo- 
graphic parlance  this  action  is  called  "restraining" 
action,  and  potassium  bromide,  a  "restrainer." 

Passing  finally  to  the  third  plate,  which  was  normally 
exposed,  but  developed  in  an  unrestrained  solution, 
comparably  with  the  second  plate  which  has  just  been 


CHEMISTRY  OF  DEVELOPMENT         6g 

under  discussion,  we  may  perhaps  see  at  once  that 
bromide  must  have  been  somehow  furnished  by  the 
plate,  since  none  was  beforehand  added  to  the  developer. 
To  explain  this  circumstance,  let  us  consider  again  our 
single  bromide  of  silver  particle,  enveloped  in  gelatine, 
and  note  what  has  happened  to  it  during  the  exposure 
to  light.  Let  us  suppose  that  the  particle  was  so  situated 
that  it  received  a  very  intense  illumination.  Because 
the  energy  of  the  light  affecting  the  particle  was  great, 
a  correspondingly  large  number  of  molecules  received 
such  a  shaking  as  to  jar  loose  one  atom  of  bromine  from 
each  molecule,  and  this  photo-chemical  action  was  able 
to  penetrate  into  the  body  of  the  particle  to  a  pro- 
portionate depth.  In  the  maximum,  all  the  silver 
bromide  molecules  of  the  particle  would  be  in  this  way 
reduced  to  sub-bromide  molecules  by  the  light.  But 
as  fast  as  the  atoms  of  bromine  were  set  free,  they  were 
immediately  taken  up  by  the  gelatine,  so  that,  prac- 
tically speaking,  none  escaped.  Therefore,  when  the 
developer  solution  reached  the  particle,  it  found, 
temporarily  combined  with  the  gelatine,  a  supply  of 
bromine  ready  at  hand  with  which  the  sodium  carbon- 
ate could  at  once  form  sodium  bromide.  This  com- 
pound, produced  right  "on  the  ground,"  so  to  speak, 
where  it  was  most  needed,  by  its  restraining  effect  upon 
the  solubility  of  any  silver  bromide  in  the  particle 
which  happened  to  escape  reduction  to  sub-salt  by  the 
light,  delayed  the  reduction  of  such  silver  bromide  by 
the  alkaline .  pyrogallate.  The  action  of  sodium  bro- 
mide is  exactly  similar  to  that  of  potassium  or  any 
other  soluble  bromide  as  a  restrainer. 

In  an  area  of  the  plate  affected  by  a  weak  illumination 
a  much  smaller  number  of  bromide  of  silver  molecules 


7o     CHEMISTRY  FOR  PHOTOGRAPHERS 

will  be  broken  down  to  sub-bromide,  and  correspond- 
ingly less  bromine  will  be  set  free,  and  thus  throughout 
the  total  extent  of  the  exposed  body  of  emulsion  the 
amount  of  reduction  to  sub-bromide  will  be  directly 
proportional  to  the  intensity  of  the  light.  Furthermore, 
it  is  important  to  observe  here  that  the  depth  in  the 
film  of  emulsion  to  which  the  photo-chemical  effect 
penetrates  is  dependent  directly  upon  the  light  intensity. 
That  is  to  say,  in  the  highlights  there  is  reduction  to 
sub-bromide  deeper  down  in  the  emulsion  than  occurs 
in  the  shadows.  Clearly,  in  some  areas  the  intensity 
of  the  light  may  be  so  low  that  its  feeble  energy  is 
wholly  absorbed  in  the  surface  layer  of  silver  bromide 
particles,  while  in  other  regions  there  will  be  an  excess 
of  energy  over  the  amount  absorbable  by  the  whole 
thickness  of  the  film  and  consequently  some  light  will 
be  transmitted.  Here  halation  will  occur  unless  the 
plate  is  suitably  backed.  But  the  silver  salt  reducible 
to  form  an  image  by  the  developer  is  not  limited  to  the 
light-reduced  sub-bromide,  for  a  secondary  reaction 
occurs  between  the  initially  reduced  silver  and  the 
neighboring  silver  bromide  that  has  escaped  partial 
reduction  by  light.  When  a  molecule  of  silver  sub- 
bromide  is  decomposed  into  its  constituents  by  the 
developer,  the  silver  is  for  the  instant  left  in  the  atomic 
condition,  the  so-called  nascent  state,  in  which  it  is  ex- 
cessively reactive.  Nascent  silver  and  silver  bromide 
react  together  immediately,  forming  silver  sub-bromide, 
which  is  reduced  by  the  developer,  and  in  this  manner 
the  particle  of  metallic  silver  resulting  may  be  much 
larger  than  would  have  been  produced  merely  from  the 
light-reduced  salt.  So,  oftentimes,  when  it  is  desired 
to  bring  up  details  in  the  shadows  without  at  the  same 


CHEMISTRY  OF  DEVELOPMENT         71 

time  carrying  the  development  of  the  highlights  too 
far,  it  is  recommended  to  take  the  plate  from  the 
developer  and  to  allow  the  action  to  continue  with  the 
amount  of  solution  which  has  soaked  into  the  emulsion. 
In  the  highlights  where  there  is  abundance  of  sub- 
bromide,  the  reducing  agent  is  quickly  used  up  and  the 
action  there  stops,  but  in  the  shadows  there  is  pro- 
portionally as  much  reducer  present  and  fewer  sub- 
bromide  molecules,  so  that  by  the  secondary  reaction 
just  described  a  considerably  greater  quantity  of  silver 
can  be  reduced  to  form  an  image  than  would  be  given 
merely  by  the  light-reduced  salt.  Too  great  stress  must 
not  be  laid  upon  this  idea  of  modifying  in  the  develop- 
ment the  gradation  established  by  the  exposure.  It  was 
formerly  believed  that  gradation  could  be  considerably 
altered  by  suitably  adjusting  the  developing  solution. 
But  it  is  now  known  that  with  a  given  plate  the  re- 
lations between  the  densities  of  the  grades  are  practic- 
ally determined  by  the  exposure,  and  can  be  little 
affected  by  the  development  process.  If  the  ratios  of 
the  intensities  of  three  tones  as  given  in  the  exposure 
are  as  1:1:3,  then  in  the  negative  produced  the 
densities  will  be  in  the  same  ratios  without  regard  to 
the  constitution  of  the  developer  or  to  the  time  of 
development.  By  the  term  "density"  is  here  meant  the 
quantity  of  silver  reduced  under  definite  conditions  by 
the  developer.  It  is  possible  to  stop  development  at 
such  a  point  that  all  three  tones  are,  so  to  speak, 
printable,  that  is,  are  sufficiently  transparent  to  print; 
or  development  can  be  carried  so  far,  for  example,  as 
to  make  density  "3"  so  opaque  that  the  ratio  of  trans- 
parency between  "2"  and  "3"  is  quite  different  from 
the  density  ratio  of  2  :  3.  In  such  a  case  the  gradation 


72   CHEMISTRY  FOR  PHOTOGRAPHERS 

obtained  in  the  print  will  be  quite  different  from  the 
original  intensities  of  tones.  This  example  will  suffi- 
ciently indicate,  perhaps,  that  the  principal  means  of 
modifying  or  controlling  gradation  lies  in  changing 
the  duration  of  development,  and  not  in  varying  the 
composition  of  the  developer.  It  is  upon  this  principle 
of  Constant  Density  Ratios  that  all  the  various  factorial, 
time,  and  tank  methods  of  development  are  based. 

So  far,  in  speaking  of  developing  agents,  we  have 
referred  to  them  in  general  terms  as  organic  compounds. 
Let  us  examine  them  a  little  more  closely,  in  order  to 
find  out  what  these  interesting  substances  are  and  how 
they  are  related  to  one  another.  When  coal  is  sub- 
jected to  distillation  for  the  purpose  of  making  coke  and 
illuminating  gas,  there  is  separated  as  one  of  the  by- 
products a  quantity  of  tarry  material  known  as  coal- 
tar.  The  substance,  once  a  waste  product,  but  now  the 
basis  of  important  industries,  is  fractionally  distilled  and 
by  this  means  there  is  obtained,  among  other  valuable 
things,  the  compound  benzene  (or  benzol) .  When  pure, 
benzene  is  a  colorless  liquid,  lighter  than  water,  and 
boiling  at  80.5  degrees  C.  A  great  many  substances 
which  are  insoluble  in  water,  such  as  fats,  resins,  etc., 
are  soluble  in  benzene,  so  that  it  is  much  used  as  a 
solvent,  but  still  more  extensively  for  the  manufacture 
of  the  so-called  benzene  derivatives,  among  which,  in 
fact,  are  most  of  our  developing  agents.  Benzene  is 
an  extremely  stable  compound,  consisting  only  of  the 
two  elements  carbon  and  hydrogen.  It  is  with  difficulty 
decomposed,  suffering  no  change  when  boiled  with 
strong  alkali,  and  only  slowly  oxidized  by  such  vigorous 
oxidizers  as  chromic  acid  and  potassium  permanganate. 
But  when  suitably  treated,  benzene  very  readily  yields 


CHEMISTRY  OF  DEVELOPMENT         73 

a  large  number  of  what  are  called  substitution  products, 
or  derivatives — substances,  that  is,  derived  from 
benzene  by  substituting  a  great  variety  of  different 
elements  and  combinations  of  elements  in  place  of 
some  or  all  of  the  hydrogen.  The  percentage  com- 
position of  benzene,  is  such  that,  taking  into  account 
also  the  density  of  its  vapor,  we  know  it  to  consist  of 
six  atoms  of  carbon  united  with  the  same  number  of 
atoms  of  hydrogen.  Employing  the  usual  chemical 
symbols,  the  formula  of  this  compound  is  consequently 
written  C6H6.  With  so  many  atoms  of  two  elements 
there  is  possible  a  number  of  different  ways  in  which 
these  atoms  might  be  joined  together,  but  it  is  fairly 
easy  to  show  that  one  method  of  representing  their 
arrangement  best  explains  the  chemical  behavior  of  the 
compound.  The  well-known  substance  acetylene  gas 
is  also  a  compound  of  hydrogen  and  carbon,  and  with  a 
percentage  composition  identical  with  that  of  benzene. 
Its  vapor  density  is  but  one-third  as  much  as  the  vapor 
density  of  benzene,  and  so  its  formula  is  written  C2H2. 
But  also  it  can  be  proved  that  in  acetylene  each  hydro- 
gen atom  is  combined  to  a  carbon  atom  and  that  the 
two  carbons  are  tied  together,  so  that  this  compound 
is  graphically  represented  thus:  H-C^C-H.  Now,  if 
acetylene  gas  be  passed  into  a  tube  which  is  kept  at  dull 
red  heat,  there  will  emerge  from  the  other  end  of  the 
tube,  not  acetylene  gas,  but  the  liquid  benzene.  So 
three  molecules  of  acetylene  are  in  this  way  combined 
together  to  form  one  molecule  of  benzene,  3  C2H2  =*= 
C6H6,  and  such  a  reaction  is  known  chemically  as 
polymerization.  Since  in  acetylene  each  hydrogen  atom 
is  joined  to  a  carbon  atom,  it  is  reasonable  to  suppose 
that  in  benzene,  the  polymer  of  acetylene,  the  same 


H 

£ 

H-C    C-H 
H-i    C-H 


Y 


Benzene 


74      CHEMISTRY  FOR  PHOTOGRAPHERS 

arrangement  holds  true.  Therefore  the  arrangement  of 
the  atoms  in  benzene,  or  its  "constitution,"  to  use  the 
chemical  term,  is  considered  to  be  representable 
graphically  as  shown  in  the  margins. 

The  six  carbons  are  all  joined  to- 
gether in  a  ring  (this  forms  the  so- 
called  "benzene  ring")  and  each  has  a 
hydrogen  atom  attached.  If  we  sup- 
pose that  the  ring  is  difficult  to  break 
apart  chemically,  but,  on  the  other 
hand,  that  the  hydrogen  atoms  can 
easily  be  taken  off  and  replaced  by 
other  elements  and  groups  of  elements, 
we  shall  see  that  this  ring  formula  very 
well  represents  the  chemical  facts 
which  we  have  previously  stated 
about  the  compound. 

When  one  hydrogen  is  taken  off  and 
in  its  place  is  put  -O-H,  we  have  the 
compound  phenol,  or  carbolic  acid,  a 
crystalline  solid.  Such  an  action  il- 
lustrates the  formation  of  "substitu- 
tion products"  or  benzene  "deriva- 
tives." If  we  also  make  a  similar  sub- 
stitution of  -O-H  on  the  opposite  side 
of  the  ring,  we  get  the  well  known 
developing  agent  hydrochinon,  or  para- 
dihydroxy benzene.  By  substituting 
three  -O-H's  on  three  adjacent  carbon 
atoms  in  the  ring,  there  is  formed  1,2, 
3,  trihydroxybenzene,  the  photog- 
rapher's good  old  stand-by,  pyrogallic 
Hydrochinon  acid,  "pyro"  for  short. 


H-CC-H 
H-4    i-H 


Phenol 

H 

i 

H-A    i-H 


H 


CHEMISTRY  OF  DEVELOPMENT 


75 


With  proper  treatment  chemically, 
hydrochinon,  which  as  a  developer 
gives  excellent  density  but  has  a 
tendency  to  "hardness"  and  lack  of 
details  in  the  shadows,  can  be  con- 
verted into  adurol,  a  more  rapid  de- 
veloper, giving  better  detail,  and  one 
that  keeps  better  in  solution  than  the 
hydrochinon  from  which  it  may  be 
derived.  Adurol  is  monochlorhydro- 
chinon,  or,  as  made  by  another  manu- 
facturer, monobromhydrochinon. 

As  examples  in  another  series  of 
substitution  products  may  be  given  the 
developers  amidol  and  metol.  These 
are  both  substituted  phenols.  Metol 
chemically  is  monomethylparamido- 
phenol.  Amidol  is  diamidophenol,  and 
ortol  is  hydrochinon  combined  with 
monomethy  lorthoamidophenol . 

If  we  particularly  note  these  seem- 
ingly awkward  chemical  names  of 
photographic  developers,  we  find  in 
many  of  them  the  prefix  "para,"  and 
in  others  "ortho."  These  prefixes,  to- 
gether with  a  third,  "meta,"  are  used 
to  signify  which  of  the  hydrogen  atoms, 
as  related  to  a  given  hydrogen,  have 
been  replaced.  Thus,  if  two  adjacent 
hydrogens  are  substituted,  as  by  -O-H 
and  -NH2,  orthoamidophenol  is  pro- 
duced, but  if  instead  the  first  and  third 
hydrogens  are  replaced,  the  compound 


H 


A 

H-C/^-O-H 
H-i    i-O-H 


Pyro 


O-H 

4 

/  ^ 

H-C    C-C1 
H-t    i-H 

V 

i-H 
Adurol 


H 


H-C    C-H 
H-C»    d-H 

sc' 

H-1VI-CH3 
Metol 


76  CHEMISTRY  FOR  PHOTOGRAPHERS 


H 


H-C    C-NH* 

H-4    i-H 

\  ^ 
C 

rsiHx 

Amidol 


H 


C     H 

/  ^     I 
H-C    C-N-CH3 


i 


-H 


V 

H 
Ortol 


H 

4 
i 

/  -^ 
H-C    C-NHi 

H-A    i-H 

\  // 

C 

H 

Orthoamidophenol 


is  metamidophenol,  and  if  the  sub- 
stitutions are  on  the  first  and  fourth 
carbon  atoms,  paramidophenol  results. 
These  three  compounds  can  actually 
be  prepared,  and  that  they  are  differ- 
ent substances,  although  possessing  the 
same  composition,  is  attested  by  the 
fact   they   have   different   properties. 
The  difference  is  in  their  constitution. 
A  very  peculiar  circumstance  about  the 
series  of  benzene  derivatives  is  that, 
whereas    the    ortho-    and    para-com- 
pounds possess  the  power  of  develop- 
ing the  latent  photographic  image,  the 
corresponding  meta-compounds  do  not 
have  this  power.  Also,  in  general,  de- 
veloping  power   is   increased   by   in- 
creased substitution.  As  an  example  of 
this    is   the   case   of   adurol    already 
mentioned,    which   is   a   more   active 
developer  than  hydrochinon,  and  yet 
differs  from  it  in  composition  and  con- 
stitution only  in  having  another  hydro- 
gen atom   replaced   by   an   atom  of 
chlorine.  Recalling  that  in  amidol  de- 
veloping   solutions    no    alkali    is    in- 
cluded and  referring  to  the  graphic 
formulas  for  metol  and  amidol  just 
given,  we  see  that  it  is  the  third  sub- 
stitution on  the  ring  which  gives  to 
amidol   so  much   greater  developing 
power   than    is   possessed    by   metol. 
Amidol  is  so  vigorous  a  reducer  that 


CHEMISTRY  OF  DEVELOPMENT         77 

its  solution  is  used  for  development  H 

simply  with  the  addition  of  sodium  o 

sulphite.  If  even  a  few  drops  of  alkali  ^ 

solution  are  added,   fog  is  likely  to  /  ^ 

supervene,  on  account  of  the  general          n     i 
reduction  of  silver  bromide  all  over  the    H  "  \  #~  NHl 
plate.  C 

A  comparison  of  all  the  developing  H 

agents  in  the  market  will  show  advan-  Metamidophenol 
tages  and  disadvantages  for  the  ama- 
teur. Some  produce  excellent  negatives,  H 
but  their  solutions  do  not  keep  well;  O 
others  have  poisonous  properties  be-                 i 
sides,   as   for    example,   metol.      Still         H-C/^-H 
others,  like  pyro,  are  prone  to  stain              1    1_H 
the  gelatine.  Some  are  rapid ;  and  some                \  // 
work  slowly.     Perhaps    it  would  be  j 
difficult  to  say  whether  pyro  or  metol-                NH* 
hydrochinon  is  the  more  used  devel-     Parami<*°Phenol 
oper,  but  one  or  the  other  of  these  is  probably  the  most 
widely  used  of  all.     It  would  be  about  equally  difficult 
to  explain  why  these  two  developers  should  be  so  popu- 
lar, since  neither  keeps  well  in  solution,  one  stains  badly 
and  the  other  is  poisonous.    Of  course,  the  merits  and 
demerits  of  developers,  like  so  many  other  things,  are 
largely  matters  of  opinion.    But,  nevertheless,  there  are 
developing  agents  which  keep  well  in  solution,  do  not 
stain  the  film,  and  are  not  especially  poisonous;  which 
also,  in  reality,  produce  quite  as  excellent  gradation 
as  pyro,  for  example,  work  with  as  much  rapidity  as 
metol,  and  are  no  more  expensive.  One  such  developer 
is  the  duratol-hydrochinon,  formulas  for  which  have 
been  extensively  published.     It  is  particularly  to  be 


78      CHEMISTRY  FOR  PHOTOGRAPHERS 

recommended  to  the  amateur  since  it  keeps  well,  almost 
indefinitely,  in  fact;  and  the  same  solution  can  be 
used  for  plates,  films,  papers,  and  lantern  slides,  by 
merely  varying  the  dilution.  Standard  recipes  for 
developers  will  be  found  in  the  Appendix. 


CHAPTER  VI 
CHEMISTRY  OF  THE  FIXING  PROCESS 


WHEN  the  operation  of  transforming  the  latent 
photographic  image  into  reduced  metallic  silver 
has  been  completed,  according  to  the  judgment  of  the 
photographer,  the  gelatine  film  upon  the  plate  con- 
tains a  considerable  number  of  different  substances. 
To  enumerate  them,  there  is  first  the  silver  which  is 
to  form  the  negative;  besides  this,  there  is  also  the  un- 
reduced remainder  of  the  unexposed  silver  bromide  and 
silver  iodide,  and  in  addition  to  these,  the  gelatine  is, 
like  a  sponge,  saturated  with  the  solution  of  all  the 
soluble  materials  which  the  developer  now  holds. 
These  last  include  the  unoxidized  reducer,  the  alkali, 
the  sulphite,  sodium  bromide,  and  sodium  iodide,  and 
the  oxidation  products  of  the  developing  agent  used  up, 
some  of  which  may  possess  the  character  of  dyes.  In 
order  to  make  a  permanent  negative  out  of  this  com- 
bination, it  is  necessary  that  everything  be  removed  so 
far  as  it  is  possible  except  the  metallic  silver  and  the 
gelatine.  Upon  taking  the  plate  from  the  developer 
solution,  the  greater  part  of  the  soluble  materials 
clinging  to  the  gelatine  should  be  washed  away  by 
rinsing  liberally  with  water.  But  primarily  it  is  re- 
quired to  get  rid  of  the  silver  salt  which  is  still  light- 
sensitive,  and  the  means  for  doing  this  must  now  be 
discussed.  By  recalling  experiments  already  performed, 
we  remember  that  silver  chloride  and  silver  bromide 
are  soluble  in  ammonium  hydroxide.  Silver  iodide  is 
very  insoluble,  but  this  reagent  has  another  disad- 

7Q 


8o      CHEMISTRY  FOR  PHOTOGRAPHERS 

vantage  which  is  serious,  namely,  that  it  has  a  bad 
effect  upon  the  gelatine.  This  is  the  reason  why 
ammonium  hydroxide  is  no  longer  much  used  as  the 
alkali  in  developer  solutions.  Another  compound  whose 
solution  in  water  is  capable  of  dissolving  all  three  of  the 
silver  halogen  salts  is  potassium  cyanide.  But,  al- 
though very  extensively  used  in  metallurgical  industry 
for  the  extraction  of  gold  and  silver  from  their  ores, 
potassium  cyanide,  on  account  of  its  extremely  poison- 
ous character,  is  not  a  reagent  that  can  safely  be 
recommended  to  the  photographer. 

Experiment  23.  Prepare  small  amounts  of  silver  chlo- 
ride, silver  bromide,  and  silver  iodide  and  treat  each  of 
them  with  a  few  cubic  centimeters  of  a  strong  solution 
of  sodium  thiosulphate  (the  "hypo"  of  the  photogra- 
pher), noting  the  solvent  effect  in  each  case.  Also  to 
a  little  cold,  dilute  sodium  thiosulphate  solution  add  a 
drop  or  two  of  silver  nitrate  in  solution  and  note  that 
the  silver  thiosulphate  at  first  precipitated  is  white. 

The  color  changes  are  due  to  the  fact  that  silver 
thiosulphate  decomposes  and  ultimately  forms  black 
silver  sulphide.  This  can  be  shown  by  heating  the 
mixture.  If  an  excess  of  the  thiosulphate  is  added 
quickly  to  a  solution  of  silver  nitrate,  the  precipitate 
of  silver  thiosulphate  at  first  formed  is  at  once  dissolved, 
producing  silver-sodium  thiosulphate,  which  is  the 
same  soluble  compound  that  is  formed  by  treating 
silver  chloride  with  thiosulphate.  Reactions: 

1 i )  Silver  bromide  +  Sodium  thiosulphate 

=  Silver-sodium  thiosulphate  +  Sodium  bromide 

(2)  Silver  nitrate  -f  Sodium  thiosulphate 

=  Silver  thiosulphate  +  Sodium  nitrate 

(3)  Silver  thiosulphate 

=»  Silver  sulphide  +  Oxygen  +  Sulphur  dioxide 

(4)  Silver  thiosulphate  -f  Sodium  thiosulphate 

=  Silver-sodium  thiosulphate 


CHEMISTRY  OF  FIXING  PROCESS       81 

Although  the  unreduced  silver  salt  remaining  after 
development  has  been  completed  is  still  sensitive  to 
light,  it  is  by  no  means  as  sensitive  as  it  was  before 
development.  If,  when  it  is  judged  that  enough  silver 
has  been  reduced,  the  plate  is  well  rinsed  in  two  or  three 
changes  of  water,  the  unaffected  silver  salt  is  so  little 
sensitive  that  it  can  be  exposed  for  a  short  time  to  the 
weak  illumination  of  an  ordinary  room  without  damage. 
Of  course  it  must  not  be  handled  in  a  strong  light.  This 
is  why  in  the  tank  development  of  roll  film  it  is  possible 
to  remove  the  film  from  the  tank  and  place  it  in  the 
fixing  bath  without  resorting  to  a  darkroom.  But 
although  the  sensitiveness  is  so  much  lessened,  never- 
theless to  make  the  negative  permanent  it  is  necessary, 
as  has  been  remarked,  to  take  away  from  the  gelatine 
everything  possible  but  the  silver.  Thus  the  "fixing" 
process  in  reality  is  a  twofold  operation,  since  it  con- 
sists in  making  the  insoluble  silver  bromide  and  silver 
iodide  over  into  soluble  silver-sodium  thiosulphate  by 
means  of  the  action  of  the  "hypo"  of  the  so-called 
"fixing  bath,"  and  afterwards  washing  out  of  the 
gelatine  all  the  soluble  salts  which  it  contains.  Ideally, 
a  negative  should  consist  solely  of  pure  silver  particles 
embedded  in  clean  gelatine.  The  more  nearly  this 
condition  is  approached  the  better  the  chemical 
quality  of  the  negative.  In  practice  it  is  not  so  simple 
a  matter  to  effect  this  desirable  state  of  things.  Some 
of  the  soluble  substances  contained  in  the  gelatine 
appear  to  "fix"  themselves  there  somewhat  as  dyes  are 
fixed  in  fabrics,  in  part,  perhaps,  adhering  to  the  silver 
image.  Certainly,  at  least,  the  staining  matter  from 
the  developer  solution  is  in  some  way  retained  by  this 
image,  as  can  be  ascertained  by  treating  such  a  dis- 


82   CHEMISTRY  FOR  PHOTOGRAPHERS 

colored  negative  with  nitric  acid.  In  this  manner  the 
silver  will  be  dissolved  out,  and  the  stain,  being  in- 
soluble in  acid  (although  it  forms  soluble  products 
with  alkali),  remains  in  the  film,  in  fact  forming,  as  it 
were,  an  image  made  of  this  dye-like  substance.  It  has 
been  claimed  that  the  reduced  silver  holds  fast  some 
of  the  unreduced  silver  bromide,  refusing  to  give  it  up 
even  to  the  thiosulphate.  Whether  or  not  this  be  true, 
if  the  decantation  process  of  washing  precipitates  is 
recalled  to  mind,  it  will  be  evident  that  here  also  the 
soluble  materials  can  be  washed  away  from  the  in- 
soluble to  any  desired  extent,  but  never  quite  entirely 
removed.  It  is  possible  and  often  desirable  to  make 
chemical  tests  of  the  wash-water  in  order  to  determine 
the  degree  to  which  the  soluble  thiosulphate  has  been 
eliminated  from  the  plate.  The  reaction  between  silver 
nitrate  and  sodium  thiosulphate  already  mentioned  can 
be  utilized  for  this  purpose. 

Experiment  24.  Make  up  a  i.o  per  cent,  sodium 
thiosulphate  solution  by  dissolving  1.57  gram  of  the 
solid  in  a  little  water  and  diluting  up  to  100  cubic 
centimeters  with  water.  Mix  this  solution  thoroughly. 
Make  a  o.  i  per  cent,  solution  by  diluting  10.0  cubic 
centimeters  of  the  i.o  per  cent,  solution  to  100  cubic 
centimeters;  and  lastly  a  o.oi  per  cent,  solution  by 
diluting  i.o  cubic  centimeter  of  i.o  per  cent,  to  100 
cubic  centimeters.  Take  10  cubic  centimeters  of  each 
of  these  three  solutions,  warm,  and  add  to  each  one 
drop  of  silver  nitrate  in  dilute  solution.  Even  the  third 
solution,  containing  so  little  thiosulphate  as  one  part 
in  ten  thousand,  gives  a  perceptible  black  precipitate 
of  silver  sulphide.  If  the  most  dilute  of  these  solutions 
be  still  further  diluted,  the  precipitate  will  be  yellowish 


CHEMISTRY  OF  FIXING  PROCESS       83 

instead  of  black,  but  will  be  distinct.  Thus  this  re- 
action furnishes  a  method  of  detecting  very  small 
amounts  of  thiosulphate.  Reaction: 

Sodium  thiosulphate  (heated)  +  Silver  nitrate 
=  Silver  sulphide  +  Silver  sulphate  +  Sodium  nitrate 
+  Sulphur  dioxide  +  Sulphur 

In  utilizing  the  reaction  of  the  preceding  experiment 
for  testing  the  thoroughness  of  the  washing,  a  plate  may 
be  taken  out  of  the  washing  tank  and  the  water  allowed 
to  drain  from  one  corner  into  a  test  tube  until  5  to  10 
cubic  centimeters  have  been  collected.  This  water  is 
next  warmed,  and  then  a  drop  of  silver  nitrate  solution 
is  added.  If  even  the  faintest  darkening  occurs,  wash- 
ing should  be  continued.  However,  we  may  remark 
that  if  the  water  used  for  washing  contains  more  than 
a  very  small  amount  of  soluble  chlorides,  the  addition 
of  silver  nitrate  will  precipitate  white  silver  chloride, 
which  will  interfere  somewhat  with  the  delicacy  of 
this  test.  In  such  a  case  use  may  be  made  of  another 
reaction,  as  in  the  following  experiment. 

Experiment  25.  Prepare  a  solution  as  follows:  In 
100  to  150  cubic  centimeters  of  distilled  water  dissolve 
i.o  gram  of  potassium  carbonate  (anhydrous)  and  o. i 
gram  of  potassium  permanganate.  Dilute  to  1,000 
cubic  centimeters  with  distilled  water.  Treat  10  cubic 
centimeters  of  each  of  the  three  solutions  of  thiosulphate 
made  up  in  the  previous  experiment  with  2,  or  3  drops 
of  this  alkaline  permanganate  solution.  The  mixture 
will  have  a  green  color,  or  blue,  if  the  thiosulphate  is 
very  dilute.  Reaction : 

Potassium  permanganate  (Pink) 

+  Potassium  carbonate  +  Sodium  thiosulphate 
=  Potassium  manganate  (Green)  +  Sodium  sulphate 
-f-  Potassium  sulphate  +  Carbon  dioxide 


84   CHEMISTRY  FOR  PHOTOGRAPHERS 

This  solution  may  be  kept  in  a  bottle  for  the  purpose 
of  testing  the  wash-water  from  plates,  films,  and  papers. 
Although  it  may  seem  an  unnecessary  complication 
to  take  the  time  to  test  the  completeness  of  washing, 
when  by  merely  allowing  the  water  to  run  long  enough 
negatives  or  prints  may  be  considered  free  from  soluble 
substances,  there  are  in  reality  sufficient  benefits, 
derived  from  making  this  test  to  warrant  the  photog- 
rapher in  adopting  it  as  a  part  of  his  regular  procedure. 
If  he  knows  from  actual  experiment  that  the  quantity 
of  thiosulphate  ("hypo")  remaining  in  his  negatives 
is  less  than  a  definite  Very  minute  amount  (and  so  for 
practical  purposes  a  harmless  amount),  he  will  never 
have  the  unhappy  experience  of  beholding  the  crystalli- 
zation of  salts  upon  his  plates,  which  not  infrequently 
befalls  the  amateur  who  takes  for  granted  the  thorough- 
ness of  his  work.  And  besides  this,  there  is  nothing  to 
be  gained,  leaving  out  of  account  the  waste  of  water, 
and  very  often  considerable  damage,  especially  to 
papers,  by  unnecessarily  prolonged  soaking.  As  we 
have  repeatedly  observed,  the  essentials  of  photo- 
graphic processes  are  chemical  in  their  nature,  and  since 
one  is  never  safe  in  taking  things  for  granted  in  regard 
to  the  results  of  chemical  operations,  it  may  be  laid 
down  as  axiomatic  that  the  particular  worker,  the 
photographer  who  makes  a  practice  of  taking  pains 
and  precautions  in  his  chemical  work,  will  produce  the 
most  technically  successful  negatives  and  prints.  In 
this  connection  there  is  a  point  which  should  not  be 
lost  sight  of,  namely,  that  no  amount  of  washing  can 
be  expected  to  take  away  insoluble  substances.  It  is 
necessary  in  the  first  instance  to  allow  the  "hypo"  to 
act  upon  the  silver  bromide  and  silver  iodide  for  a 


CHEMISTRY  OF  FIXING  PROCESS        85 

sufficient  time  so  that  the  conversion  of  these  water- 
insoluble  substances  into  the  water-soluble  double 
thiosulphate  of  silver  and  sodium  may  be  effected 
completely.  By  allowing  the  plate  to  stay  in  the  fixing 
bath  for  at  least  ten  minutes  after  the  creamy  silver 
halogen  salts  have  disappeared,  one  can  always  be 
assured  of  thorough  "fixing,"  provided  the  thiosulphate 
solution  is  reasonably  fresh,  and  not  approaching  ex- 
haustion. Much  of  the  amateur's  trouble  in  the  way 
of  poor  negatives  and  prints  is  doubtless  traceable  to 
the  overworking  of  the  fixing  solution,  not  to  say  also 
of  the  developer.  It  may  be  that  a  supply  of  develop- 
ing solution  and  a  stock  of  "hypo"  will  last  for  a  greater 
length  of  time  measured  in  months,  and  will  develop 
and  fix  more  negatives,  if  used  until  the  last  vestiges 
of  chemical  activity  have  been  exhausted,  but  such  a 
proceeding  is  not  conducive  to  excellence  and  uni- 
formity and  permanence  of  results.  Neither  is  such  a 
method  really  economical,  for  the  saving  in  developer 
and  "hypo"  is  far  overbalanced  by  waste  of  the  much 
more  expensive  plates,  films,  and  papers  that  are 
spoiled.  The  best  practice  for  the  amateur  to  adopt  is 
first  never  to  use  over  again  a  solution  which  has  once 
developed  a  plate  or  film,  and  second  to  mix  enough 
fresh,  plain  thiosulphate  "fixing  bath"  for  each  batch 
of  plates  or  films,  throwing  it  away  when  they  are 
done.  There  is  no  better  fixing  solution  for  plates  and 
films  than  plain  "hypo,"  if  one  of  the  non-staining 
modern  developing  agents  has  been  used.  Besides  this, 
if  a  tendency  to  discoloration  appears,  the  stain  can  be 
bleached  by  adding  a  little  sodium  sulphite,  or,  even 
better,  the  potassium  disulphite  which  is  commonly 
called  "metabisulphite." 


86   CHEMISTRY  FOR  PHOTOGRAPHERS 

The  "acid"  fixing  bath  is  simply  a  solution  of  "hypo" 
to  which  is  added  sulphite  and  enough  acid  to  liberate 
some  sulphurous  acid  from  the  sulphite,  but  not  suffi- 
cient to  decompose  the  thiosulphate.  Recalling  the  use 
of  sodium  sulphite  and  potassium  metabisulphite  in  de- 
veloping solutions  where  they  are  commonly  called  "pre- 
servatives," we  may  remember  that  their  function  is 
equally  toprevent  the  formation  of  dyes  which  would  pro- 
duce discoloration.  So  in  this  sense  they  are  "bleachers." 
In  the  fixing  bath  they  perform  the  same  service,  pre- 
venting the  gelatine  from  becoming  discolored  by  bleach- 
ing the  dye  which  has  been  formed  in  the  developer.  It 
is  in  fact  the  sulphur  dioxide,  or  sulphurous  acid,  as  the 
solution  of  this  gas  in  water  is  called,  which  is  the  effec- 
tive bleaching  agent.  The  acid  used  to  set  free  the  sul- 
phurous acid  from  sodium  sulphite  is  ordinarily  citric 
or  tartaric  acid,  but  sometimes  very  dilute  sulphuric 
acid  is  employed.  The  reactions  are  as  follows : 

Sodium  sulphite  +  Citric  acid 

=  Sulphurous  acid  +  Sodium  citrate 
Sodium  sulphite  +  Sulphuric  acid 

=  Sulphurous  acid  +  Sodium  sulphate 

Sulphurous  acid  is  a  very  unstable  compound  and  is 
known  as  such  only  in  solution.  Even  there  it  is  con- 
tinually breaking  down  into  its  constituents,  sulphur 
dioxide  gas  and  water,  and  this  is  the  reason  why  its 
solutions  have  such  a  pungent  odor.  Like  its  sodium 
salt  it  is  a  reducing  agent,  and  it  is  upon  this  fact  that 
its  efficiency  in  preventing  stain  depends.  All  dyes,  on 
becoming  reduced,  form  colorless  products,  and  it  is 
this  action  which  is  called  bleaching.  Potassium  metabi- 
sulphite, or  "disulphite"  as  it  really  should  be  named, 
produces  similar  results  by  reason  of  the  fact  that  its 


CHEMISTRY  OF  FIXING  PROCESS       87 

solution  is,  in  effect,  the  same  as  a  sulphite  solution  to 
which  acid  has  been  added.  This  compound  is,  as  it 
were,  potassium  sulphite  containing  extra  sulphur 
dioxide  which  it  easily  gives  up.  Therefore,  to  re- 
capitulate, the  addition  of  acid  to  sodium  or  potassium 
sulphite,  or  to  disulphite,  produces  the  sodium  or 
potassium  salt  of  that  acid  and  sulphurous  acid,  if  the 
action  goes  on  in  solution.  But,  as  we  have  said, 
sulphurous  acid  spontaneously  decomposes  into  water 
and  sulphur  dioxide,  so  that  the  reaction  given  above 
might  be  written  quite  as  well : 

Sodium  sulphite  +  Sulphuric  acid 

=  Sulphur  dioxide  +  Water  +  Sodium  sulphate 

Next  we  must  inquire  into  the  effect  of  acid  upon 
sodium  thiosulphate. 

Experiment  26.  Add  a  little  dilute  sulphuric  acid  to 
5  cubic  centimeters  of  a  fairly  concentrated  solution  of 
sodium  thiosulphate,  note  the  yellowish-white  precipi- 
tate, which  is  sulphur;  and  cautiously  smell  the  odor  of 
the  sulphur  dioxide  gas  which  is  given  off.  Try  the  effect 
of  the  same  acid  on  more  dilute  thiosulphate  solutions, 
and  observe  that  similar  results  follow.  Dissolve  a  crys- 
tal or  two  of  citric  acid  in  water  and  try  the  effect  of  its 
dilute  solution  upon  thiosulphate.  Similarly  experiment 
with  dilute  acetic  acid,  observing  that  with  these  so- 
called  organic  acids  also  the  separation  of  sulphur  and 
evolution  of  sulphur  dioxide  occur.  Reactions : 

i .  Sodium  thiosulphate  +  Sulphuric  acid 

=  Sodium  sulphate  +  Sulphur  +  Sulphur  dioxide 

+  Water 
i.  Sodium  thiosulphate  +  Citric  acid 

=   Sodium  citrate   +   Sulphur    +  Sulphur  dioxide 

+  Water 
3 .  Sodium  thiosulphate  +  Acetic  acid 

=  Sodium  acetate   +  Sulphur  +  Sulphur  dioxide 

+  Water 


88  CHEMISTRY  FOR  PHOTOGRAPHERS 

Next  dissolve  0.2  or  0.3  gram  of  sodium  sulphite  in 
about  5  cubic  centimeters  of  water  and  add  several 
drops  of  dilute  sulphuric  acid,  avoiding  an  excess  of 
the  acid.  This  solution  will  now  contain,  referring  to 
the  reaction  above,  sodium  sulphate  and  sulphurous 
acid,  besides  sodium  sulphite,  since  the  last-named 
compound  is  in  excess.  Treat  a  little  thiosulphate  in 
solution  with  some  of  this  sulphurous  acid  and  note  that 
there  is  no  deposition  of  sulphur. 

Since  the  sulphur  set  free,  when  thiosulphate  is 
acidified,  comes  from  the  spontaneous  decomposition 
of  the  thiosulphuric  acid  formed,  it  is  plain  that  sul- 
phurous acid  is  unable  to  react  with  sodium  thiosul- 
phate to  liberate  thiosulphuric  acid.  Therefore  if  we 
make  a  mixture  of  sodium  sulphite  and  thiosulphate 
in  the  same  solution  and  add  to  it  a  less  amount  of  acid 
than  would  be  equivalent  to  the  sulphite,  we  should 
expect  the  acid  to  interact  with  the  sulphite,  forming 
sulphurous  acid  in  the  solution,  and,  since  in  this  way 
the  danger  of  forming  thiosulphuric  acid  is  removed, 
there  should  be  no  separation  of  sulphur.  An  experi- 
ment will  show  that  this  is  the  case,  and  it  is  in  such  a 
manner  that  the  "acid"  fixing  baths  are  made  up. 
Whenever  sulphur  is  precipitated  from  such  a  bath,  it  is 
evidence  of  the  presence  in  the  solution  of  some  free 
acid  other  than  sulphurous  acid.  It  is  this  separation 
of  sulphur  from  thiosulphate  which  is  one  of  the  causes 
of  lack  of  permanency  in  negatives  and  prints  that 
have  been  imperfectly  washed,  so  that  any  agency 
which  is  capable  of  decomposing  thiosulphate  should 
be  used  in  photographic  solutions  with  care  and  in- 
telligence. For  the  making  of  negatives,  at  least,  except 
possibly  in  very  hot  weather  or  in  the  tropics,  there  is 


CHEMISTRY  OF  FIXING  PROCESS       8q 

no  fixing  bath  better  than  one  of  plain  "hypo,"  to 
reiterate  a  statement  already  made.  At  most  there 
need  be  added  to  this  nothing  more  than  a  little  potas- 
sium metabisulphite.  If  there  is  real  danger  that  the 
gelatine  will  become  too  much  softened,  then  a  harden- 
ing agent  should  be  employed  in  an  acid  "hypo"  bath. 
One  of  the  best  of  these  is  the  "acid  fixing  and  hardening 
bath"  which  has  been  recommended  for  a  long  time 
past  with  Cramer  plates.  The  hardening  consists  in 
rendering  the  gelatine  more  insoluble  in  water  and  for 
this  purpose  a  chromium  salt  is  used,  generally  "chrome 
alum,"  or,  chemically,  potassium  chromium  sulphate. 
By  treatment  with  chrome  alum  the  solubility  of  gela- 
tine is  very  greatly  diminished,  so  much  so  that  after  a 
sufficiently  prolonged  application  the  plate  can  be 
washed  even  with  hot  water  without  danger  of  melting. 
There  is  no  special  advantage  in  hardening  the  film  to 
such  an  extent,  but  on  the  other  hand  it  will  be  dis- 
tinctly disadvantageous  if  any  after-treatment  is  to  be 
used.  Another  disadvantage  of  the  acid  fixing  bath 
is  that  it  is  often  liable  to  be  used  after  its  efficiency  has 
become  too  much  impaired.  For  a  plain  "hypo"  fixing 
bath  for  general  use,  dissolve  100  grams  of  the  crys- 
tallized salt  in  enough  warm  water  and  dilute  to  500 
cubic  centimeters.  Since  sodium  thiosulphate  crys- 
tallizes with  5  molecules  of  water,  this  will  make  a 
solution  a  little  stronger  than  12  per  cent,  in  sodium 
thiosulphate.  This  is  for  fixing  plates  and  films;  for 
prints,  use  75  grams,  instead  of  100  grams,  of  "hypo," 
and  12.5  grams  of  potassium  disulphite  (metabisulphite) . 
The  same  amount  of  the  latter  compound  can  be  added 
to  the  bath  for  plates  also. 

There  has  at  various  times  been  suggested  a  number 


qo  CHEMISTRY  FOR  PHOTOGRAPHERS 

of  "hypo  eliminators,"  but  none  of  them  is  to  be  recom- 
mended on  account  of  the  lack  of  permanency  in  the 
negatives  so  prepared.  They  all  work  upon  the  same 
principle,  the  oxidation  of  thiosulphate  either  to 
tetrathionate  or  to  sulphate.  Among  these  oxidizing 
agents  are  hypochlorites,  hydrogen  dioxide,  potassium 
permanganate,  percarbonates  and  persulphates.  It 
is  quite  as  easy  to  wash  soluble  thiosulphate  out  of 
the  gelatine  as  any  other  soluble  products,  and  no 
"hypo  eliminator"  can  eliminate  the  process  of  washing, 
so  that  there  seems  little  to  be  gained  by  adding  an 
extra  operation  to  the  chemical  processes  of  the  photo- 
graphic art. 

After  the  removal  of  all  dissolved  materials  from  the 
gelatine  film,  the  last  step  in  the  making  of  a  negative, 
unless  some  modifications  are  to  be  made  upon  it,  is  the 
drying  out  of  the  water.  The  plates  may  be  set  in  a 
drying  rack  and  put  in  a  cool,  airy  place  until  com- 
pletely dried.  The  drying-room  should  be  as  free  as 
may  be  from  dust,  and  there  ought  to  be  no  consid- 
erable change  in  temperature  during  the  drying.  The 
more  rapidly  drying  takes  place,  therefore,  the  safer 
for  the  negatives.  When  a  plate  dries  slowly  until 
partly  done  and  then,  with  a  wet  patch  in  the  middle,  is 
finished  at  a  different  temperature,  there  may  be  a 
marked  difference  between  the  density  of  the  portion 
that  dried  at  the  one  temperature  and  that  which 
dried  at  the  other.  In  this  way  the  printing  quality  of 
the  negative  will  be  spoiled.  A  mistake  which  is 
common  in  many  plate-racks  is  that  the  grooves  are  too 
close  together.  Plates  are  brought  so  near  to  each 
other  that  free  circulation  of  air  between  them  is 
impeded  and  uniform  drying  is  rendered  difficult.  The 


CHEMISTRY  OF  FIXING  PROCESS        91 

least  distance  which  it  is  advisable  to  have  between 
the  plates  is  about  one  inch.  A  rapid  method  of  drying 
which  has  been  devised  is  to  immerse  the  plate  in  a 
very  concentrated  solution  of  sodium  or  potassium  car- 
bonate. Such  a  solution  has  a  strong  affinity  for  water, 
so  that  the  water  in  the  gelatine  is  at  once  seized  by 
the  sodium  or  potassium  carbonate,  and  in  a  short 
time  the  plate  can  be  removed  from  the  bath  quite  dry. 
The  process  must  be  used  with  care,  however,  as  the 
film  sometimes  leaves  the  glass.  It  must  not  be  used 
with  films,  which  will  be  ruined  by  distortion.  When  it 
is  desired  to  dry  a  negative  rapidly,  it  may,  after  drain- 
ing for  a  few  minutes,  be  placed  in  a  tray  containing 
ordinary  alcohol.  An  immersion  of  5  to  10  minutes 
allows  the  water  in  the  gelatine  to  become  mixed  by 
diffusion  with  alcohol,  and  thus  largely  replaced  by 
the  alcohol,  after  which  the  negative  will  dry  very 
quickly  owing  to  the  rapid  evaporation. 

Upon  the  skill  with  which  the  principles  we  have  so  far 
considered  are  applied  depends  the  success  of  the  pho- 
tographer, whether  he  be  specially  interested  in  straight 
record  photography  or  in  pictorial  work.  Although  it 
is  at  the  same  time  true  that  the  negative  is  merely 
the  means  to  an  end,  viz.,  the  print,  nevertheless  it  is  in 
the  wonderfully  modulated  silver  deposits  of  the  nega- 
tive that  are  recorded  the  gradations  of  light  and  shade 
of  which  the  object  depicted,  speaking  photographic- 
ally, consists.  No  matter  what  modifications  may  be 
later  introduced  in  the  print  to  suit  the  desires  and 
taste  of  the  worker,  the  basis  of  all  good  photography 
of  whatever  sort  is  careful  attention  to  the  psychological, 
optical,  and  chemical  laws,  which,  when  understandingly 
obeyed,  make  the  negative  a  truthful  and  yet  flexible 


QZ   CHEMISTRY  FOR  PHOTOGRAPHERS 

medium  between  the  object  photographed  and  the 
picture  which  is  finally  obtained.  For  this  reason  the 
reader  is  urged,  if  he  wishes  to  secure  the  greatest 
pleasure  and  benefit  from  his  excursions  in  the  photo- 
graphic realm,  to  pay  particular  heed  to  his  chemical 
work.  The  cultivation  of  accurate  and  intelligent 
methods  in  field,  studio,  and  darkroom  will  lead  even 
the  tyro  farther  into  the  "mysteries"  than  he  may 
anticipate. 


CHAPTER  VII 
AFTER-TREATMENT  OF  THE  NEGATIVE 


WHEN  the  exposure  of  the  plate,  upon  an  object 
whose  range  of  gradation  is  not  too  great,  has 
been  confined  within  the  limits  of  the  latitude  of  the 
emulsion,  and  its  development  has  been  carried  out 
to  the  proper  degree  of  opacity,  nothing  remains  but  to 
select  such  a  printing  medium  as  will  give  the  effects 
desired  in  the  finished  picture.  But  more  or  less 
frequently  it  will  happen  that  these  ideal  conditions  will 
not  all  be  fulfilled.  Either  the  original  contrasts  of  the 
object  photographed  were  too  great,  or  under-  or  over- 
exposure  has  been  given,  or  sometimes  the  duration 
of  development  is  not  properly  regulated.  If  the  di- 
vergence from  these  normal  conditions  is  more  than  a 
comparatively  moderate  amount,  there  is  no  better 
advice  to  give  the  photographer  than  to  make  another 
trial  and  use  greater  care.  Not  always,  of  course,  can 
the  exposure  be  repeated,  and  when  this  is  the  case,  if  a 
print  must  be  had,  it  is  possible  to  modify  the  negative 
to  a  considerable  extent  by  the  processes  of  intensifica- 
tion and  (as  it  is  called)  reduction.  In  general,  both 
kinds  of  operation  consist  in  the  beginning  of  the  same 
type  of  reaction,  namely,  the  oxidation  of  a  part  of  the 
silver  of  which  the  image  is  composed. 

Some  intensification  methods  do  not  follow  this  rule, 
consisting,  instead,  of  the  suitably-controlled  reduction 
of  silver  from  the  solution  applied  to  the  plate.  Let 
us  consider  first  the  mercury  intensifies. 

Experiment  27.    Give  a  brief  exposure  to  each  of 

Q3 


94  CHEMISTRY  FOR  PHOTOGRAPHERS 

three  small  plates  and  develop,  fix,  and  wash  them  as 
usual.  Make  up  100  cubic  centimeters  of  a  two  per 
cent,  or  three  per  cent,  solution  of  mercuric  chloride 
("corrosive  sublimate,"  a  violent  poison  and  anti- 
septic). Immerse  one  end  of  each  prepared  negative  in 
this  solution  until  completely  bleached,  observing  the 
change  from  the  back  by  transmitted  light.  Wash  the 
three  plates  thoroughly  for  ten  to  fifteen  minutes  and 
then  treat  them  severally  as  follows.  Take  50  cubic 
centimeters  of  water  in  a  small  tray,  add  a  few  drops 
of  ammonium  hydroxide,  and  put  one  of  the  plates  into 
this  solution.  An  immediate  blackening  of  the  bleached 
image  will  be  noted.  Dissolve  5  grams,  of  sodium 
thiosulphate  in  25  to  30  cubic  centimeters  of  water, 
dilute  to  50  and  treat  the  second  bleached  plate  with 
this  solution.  Here  again  will  occur  a  darkening  of 
the  whitened  portion.  To  blacken  the  third  plate,  use 
a  regular  developer  solution.  The  three  plates  may  now 
be  well  washed  and  dried,  and  the  opacities  of  the 
intensified  and  unintensified  parts  compared. 

In  explaining  the  chemical  reactions  which  take 
place  in  this  operation,  we  may  first  observe  that  there 
are  two  classes  of  mercury  compounds  representing 
two  conditions  of  oxidation,  namely,  the  mercuric  salts, 
the  higher  condition,  and  the  mercurous,  the  lower. 
Mercuric  salts  can  be  changed,  by  the  action  of  re- 
ducing agents,  to  mercurous;  and  vice  versa,  the  mer- 
curous are  oxidizable  to  mercuric  compounds.  Further, 
mercurous  compounds  are  completely  reducible  to 
elementary  mercury,  and,  when  this  reduction  takes 
place  from  a  solution  of  the  mercury  salt,  the  resulting 
mercury  is  in  a  very  finely  divided  condition,  and, 
since  its  absorption  of  all  wave-lengths  of  light  is  very 


AFTER-TREATMENT  OF  NEGATIVE      95 

complete,  its  color  is  an  intense  black.  Now,  as  we  saw 
in  the  chapter  on  the  photo-chemistry  of  silver  salts, 
the  silver  of  which  the  negative  image  consists  is  also 
in  a  fine  state  of  division,  and  in  this  condition  is  a 
good  reducing  agent.  Consequently,  when  mercuric 
chloride  solution  is  applied  to  the  negative,  the  silver 
reduces  an  equivalent  amount  of  the  mercuric  chloride 
to  mercurous  chloride  and  is  at  the  same  time  itself 
oxidized  to  silver  chloride.  Thus  the  bleached  image 
consists  of  a  mixture  of  equivalent  quantities  of  silver 
chloride  and  mercurous  chloride,  both  being  insoluble 
white  substances.  There  may  also  be  some  unchanged 
silver.  Reaction: 

Silver  +  Mercuric  chloride 

=  Silver  chloride  +  Mercurous  chloride 

The  character  of  the  mercury  compound  will  be  seen 
from 

Experiment  28.  Prepare  a  few  cubic  centimeters  of 
sodium  sulphite  solution,  acidify  with  a  drop  or  two 
of  dilute  sulphuric  acid,  and  add  to  it  about  10  cubic 
centimeters  of  two  per  cent,  mercuric  chloride  solution, 
observing  the  white  precipitate  of  mercurous  chloride, 
which  is  formed  by  the  reduction  of  the  mercuric  salt. 
(If  the  mixture  were  now  to  be  heated,  mercurous 
chloride  would  be  further  reduced  to  elementary 
mercury,  but  this  is  not  desirable  in  the  present  ex- 
periment) .  Allow  the  mercurous  chloride  to  settle,  wash 
it  by  decantation,  and  keep  it  for  use  in  a  subsequent 
experiment.  Reactions  : 

(i)  Sodium  sulphite  +  Sulphuric  acid 

=  Sulphurous  acid  +  Sodium  sulphate 
(a)  Mercuric  chloride  +  Sulphurous  acid  +  Water 
=  Mercurous  chloride  +  Sulphuric  acid  +  Hydrochloric 

acid 


q6      CHEMISTRY  FOR  PHOTOGRAPHERS 

Now,  of  the  three  bleached  plates  the  first  was  treated 
with  diluted  ammonium  hydroxide.  That  is  to  say,  the 
mixture  of  silver  and  mercurous  chlorides  was  acted 
upon  by  the  ammonium  hydroxide  in  such  a  way  as 
to  form  a  black  product.  By  reference  to  the  behavior 
of  silver  chloride  with  ammonium  hydroxide,  we  re- 
member that  a  soluble  product  is  formed,  the  reaction 
being  thus : 

Silver  chloride  +  Ammonium  hydroxide 

=  Silver-ammonia  chloride  +  Water 

The  action  of  ammonium  hydroxide  upon  mercurous 
chloride  may  be  shown  as  follows : 

Experiment  29.  Take  a  portion  of  the  mercurous 
chloride  prepared  in  Experiment  27  and  treat  it  with  a 
few  drops  of  dilute  ammonium  hydroxide,  observing 
the  instant  blackening  which  occurs.  There  are  formed 
as  insoluble  products  in  this  reaction  elementary 
mercury,  which  is  black  in  a  fine  state  of  division,  and 
mercury-ammonium  chloride,  which  is  white.  Reaction : 

Mercurous  chloride  -f-  Ammonium  hydroxide 
=  Mercury  +  Mercury-ammonium  chloride  +  Ammo- 
nium chloride  +  Water 

The  mixture  of  these  black  and  white  insoluble  sub- 
stances is  black.  Thus  in  this  first  process  of  intensifica- 
tion, the  silver  image  is  changed  initially  to  one  com- 
posed of  silver,  silver  chloride,  and  mercurous  chloride. 
By  the  ammonium  hydroxide  the  silver  chloride  is 
rendered  soluble  and  the  mercurous  chloride  is  converted 
to  an  insoluble  black  mixture  of  mercury  and  a  complex 
mercury  compound.  So,  when  the  first  plate  is  finally 
washed,  the  image  remaining  consists  of  silver  and 
mercury,  plus  the  small  amount  of  white  complex 
substance.  The  color  of  the  intensified  image  formed  in 


AFTER-TREATMENT  OF  NEGATIVE      Q7 

this  way  is  generally  a  dark  brown,  which  makes  the 
process,  when  successfully  carried  out,  suitable  for 
securing  good  prints  from  the  negative,  but  the  method 
has  the  disadvantage  of  being  somewhat  uncertain  in  its 
results. 

In  the  case  of  the  second  plate  with  which  we  have 
experimented,  the  mixture  of  silver  and  mercurous 
chlorides  was  acted  upon  by  sodium  thiosulphate. 
Here  again  it  becomes  immediately  clear  that  silver 
will  be  removed  from  the  image,  since  silver  chloride 
and  sodium  thiosulphate  form  soluble  silver-sodium 
thiosulphate,  carried  away  in  the  wash-water.  Re- 
action : 

Silver  chloride  -f-  Sodium  thiosulphate 

=  Silver-sodium  thiosulphate 

As  for  the  action  of  the  sodium  thiosulphate  upon 
mercurous  chloride,  we  already  know  that  not  only  is 
thiosulphate  a  mild  reducing  agent,  but  that  mercurous 
chloride  is  an  oxidizer,  and  therefore  the  result  of  their 
interaction  will  be  to  form  elementary  mercury  by  the 
reduction  of  the  one  and  an  oxidation  product  of  the 
other,  thus: 

Mercurous  chloride  +  Sodium  thiosulphate 
=  Mercury  +  Sodium  chloride  +  Sodium  tetrathionate 

Therefore,  in  the  second  method  also,  by  use  of 
'  'hypo, ' '  a  portion  of  the  silver  image  is  removed,  but  is  at 
the  same  time  replaced  by  a  larger  quantity  of  black 
mercury.  This  removal  of  silver  from  the  image  is 
avoided  in  the  third  method  in  which  developer  solution 
is  used,  as  the  organic  reducer  acts  upon  both  the  silver 
chloride  and  the  mercurous  chloride,  reducing  them  to 
elementary  silver  and  elementary  mercury  respectively. 
Another  method  of  intensification,  whereby  mercury 


q8      CHEMISTRY  FOR  PHOTOGRAPHERS 

is  added  to  the  silver,  consists  in  treating  the  plate  with 
a  solution  containing  mercuric  iodide  and  concentrated 
sodium  sulphite.  Here  silver  is  oxidized  to  silver  iodide, 
while  mercuric  iodide  is  reduced  to  the  mercurous  salt, 
but  immediately  the  last  is  reduced  by  the  sodium 
sulphite  to  mercury.  The  plate  is  then  treated  with  a 
developer,  by  which  silver  iodide  is  reduced  to  silver. 
If  this  last  step  is  omitted,  the  intensified  image  is 
liable  to  fade.  Reactions: 

(1)  Silver  +  Mercuric  iodide 

=  Silver  iodide  +  Mercurous  iodide 

(2)  Mercurous  iodide  +  Sodium  sulphite  +  Water 

=  Mercury  +  Sodium  sulphate  +  Hydriodic  acid 

(3)  Silver  iodide  +  Organic  reducer 

=  Silver  +  Oxidation  products  of  organic  reducer 

An  intensification  process  in  which  metallic  silver  is 
added  to  the  silver  image  is  one  which  is  known  as  the 
"Wellington  intensifier."  In  this  a  solution  containing 
silver,  an  organic  reducing  agent,  and  an  alkali  is  so 
adjusted  that  it  is  just  on  the  point  of  precipitating 
elementary  silver.  Under  these  circumstances,  when 
the  solution  is  applied  to  a  negative,  the  particles  of 
silver  already  composing  the  image  act  as  nuclei  upon 
which  the  silver  reducing  from  the  solution  can  deposit. 
Thus  the  image  can  be  built  up  to  any  extent  desired. 
This  action  can  be  illustrated  by  taking  a  little  satu- 
rated solution,  for  example,  of  copper  sulphate  ("blue 
vitriol"),  putting  into  it  a  small  crystal  of  the  same 
substance,  and  allowing  crystallization  to  take  place 
slowly.  The  introduced  crystal  will  gradually  increase 
in  size,  and  possibly  no  other  crystals  will  form,  at  least 
for  a  time.  It  is  easier,  so  to  say,  for  the  copper  sulphate 
in  the  solution  to  separate  out  upon  an  already  formed 
crystal  than  to  start  new  ones.  In  a  similar  way,  it  is 


AFTER-TREATMENT  OF  NEGATIVE      99 

also  easier  for  the  silver  separating  by  reduction  from 
the  intensifying  solution  to  deposit  upon  the  silver 
particles  already  present  in  the  gelatine  than  to  com- 
mence the  formation  of  new  particles.  To  experiment 
with  this  process,  make  up  the  following  solutions : 

1.  Ammonium  sulphocyanide        4.0  grams 
Sodium  thiosulphate 4.0  grams 

Water,  to 25.0  cubic  centimeters 

2.  Silver  nitrate 2.5  grams 

Water,  to 25.0  cubic  centimeters 

Take  1 5  cubic  centimeters  of  Solution  i  and  add  to  it 
slowly  and  stirring  constantly  15  cubic  centimeters  of 
the  second.  The  action  of  a  silver  solution  upon  am- 
monium sulphocyanide  is,  as  may  be  quickly  ascertained 
by  experiment,  to  precipitate  white  silver  sulpho- 
cyanide. But  in  the  presence  of  sodium  thiosulphate 
this  precipitation  is  prevented,  owing  to  the  formation 
of  the  soluble  silver-sodium  thiosulphate,  until  the 
quantity  of  thiosulphate  present  has  been  used  up, 
when  immediately  silver  sulphocyanide  will  commence 
to  come  down.  Reaction: 

Silver  nitrate  +  Ammonium  sulphocyanide 

=  Silver  sulphocyanide  +  Ammonium  nitrate 

If  a  permanent  precipitate  is  formed  by  the  mixture  of 
the  two  solutions,  add  cautiously,  a  few  drops  at  a  time, 
more  of  the  first  until  the  precipitate  is  just  dissolved. 
If,  on  the  other  hand,  no  precipitate  has  been  formed, 
add  more  silver  nitrate  carefully  until  a  slight  per- 
manent precipitate  appears  and  then  barely  dissolve 
it  with  a  drop  or  two  of  Number  i .  Next  dissolve  in  ^ 
cubic  centimeters  of  water  0.4  gram  of  sodium  sulphite 
and  0.2  gram  of  pyrogallic  acid,  add  2  cubic  centimeters 
of  a  ten  per  cent,  solution  of  ammonium  hydroxide, 


ioo    CHEMISTRY  FOR  PHOTOGRAPHERS 

and  pour  this  mixture  into  the  liquid  already  prepared. 
Put  a  dry  negative  that  needs  intensification  into  a 
clean  tray  and  cover  it  with  this  solution,  rocking  the 
tray  until  sufficient  density  has  been  obtained.  Then 
put  the  plate  into  a  fresh  fixing  bath  for  five  minutes, 
wash  thoroughly,  and  dry  it.  In  explaining  the  chemical 
reaction  of  this  process,  we  may  assume  that  we  have 
silver  sulphocyanide  in  solution  mixed  with  the  organic 
reducing  agent  and  in  contact  with  the  metallic  silver  of 
the  image.  Under  these  circumstances  the  silver,  which 
is  gradually  reduced,  deposits  upon  the  metal  in  the  gela- 
tine and  not  elsewhere,  and  so  there  results  a  building 
up  of  the  image  by  an  accumulation  of  silver.  Reaction : 

Silver  sulphocyanide  +  Pyrogallic  acid  +  Water 

=  Silver  +  Sulphocyanic  acid  +  Acetic  acid 
+  Oxalic  acid 

There  are  two  other  methods  of  increasing  the  opacity 
of  negatives  which  are  much  employed,  one  using 
uranium  and  the  other,  chromium  salts.  The  uranium 
intensifier  has  the  advantage  that  the  operation  can 
be  carried  on  in  a  single  solution,  after  which  the 
plate  must  be  thoroughly  washed,  but  the  negative 
must  in  the  first  instance  have  been  very  completely 
freed  from  thiosulphate,  else  the  intensification  will  not 
be  uniform  and  stains  will  appear. 

Experiment  30.  Mix  a  few  cubic  centimeters  of  very 
dilute  solutions  of  potassium  ferrocyanide  and  uranium 
nitrate  and  observe  the  blood-red  coloration  that  is 
developed.  Uranium  ferrocyanide  is  formed.  Compare 
Experiment  8.  Reaction : 

Uranium  nitrate  +  Potassium  ferrocyanide 

=  Uranium  ferrocyanide  +  Potassium  nitrate 

The  solution  used  in  this  process  contains,  in  addition 


AFTER-TREATMENT  OF  NEGATIVE     101 

to  the  uranium  nitrate,  potassium  ferricyanide  acidified 
with  acetic  acid.  The  latter  salt,  in  the  presence  of  the 
acid,  is  an  oxidizing  agent,  and  thus,  when  applied  to 
the  silver  image,  oxidizes  silver  to  silver  oxide,  which 
with  the  excess  of  acetic  acid  forms  silver  acetate.  But 
the  ferricyanide  is  simultaneously  reduced  to  ferro- 
cyanide,  whereupon  there  occurs  the  reaction  just 
mentioned  in  which  insoluble  red  uranium  ferrocyanide 
is  produced.  Therefore  some  of  the  silver,  in  being 
oxidized,  is  rendered  soluble  and  taken  away,  but  is 
replaced  by  the  very  non-actinic  and  insoluble  uranium 
compound.  Reactions: 

(i)  Potassium  ferricyanide  +  Acetic  acid 

=  Ferricyanic  acid  +  Potassium  acetate 
(i)  Silver  -f-  Ferricyanic  acid  +  Water 

=  Silver  oxide  +  Ferrocyanic  acid 

(3)  Silver  oxide  +  Acetic  acid 

=  Silver  acetate  +  Water 

(4)  Uranium  nitrate  +  Ferrocyanic  acid 

=  Uranium  ferrocyanide  +  Nitric  acid 

To  prepare  the  uranium  intensifier,  make  equal  quan- 
tities, say  100  cubic  centimeters  each,  of  a  one  per 
cent,  solution  of  uranium  nitrate,  and  of  one  per  cent, 
potassium  ferricyanide  solution  containing  twenty  per 
cent,  of  acetic  acid  (glacial).  These  two  solutions  are 
to  be  united  and  may  then  be  used  as  desired.  If  by 
leaving  the  plate  too  long  in  this  intensifier  the  opacity 
is  too  much  increased,  it  may  be  diminished  somewhat 
by  long  washing  in  running  water,  especially  if  the 
water  be  hard.  Ordinarily  the  intensified  negative 
should  be  washed  for  about  10  minutes. 

The  chromium  intensifier  is  one  of  the  simplest  to 
use,  in  addition  to  which  the  operation  can  be  repeated 
as  often  as  desired.  For  these  reasons  it  is  perhaps  more 


102    CHEMISTRY  FOR  PHOTOGRAPHERS 

satisfactory  than  any  other  method,  particularly  for 
the  beginner,  if  he  uses  the  "tabloid"  chromium  in- 
tensifier,  which  is  especially  convenient.  The  plate  is 
bleached  in  a  solution  of  one  of  the  tabloids,  it  is  washed 
for  a  quarter  of  an  hour,  and  it  is  then  redeveloped  in  the 
usual  developing  solution,  the  whole  operation  proceed- 
ing in  daylight.  Solutions  for  this  method  may  be 
prepared  as  follows: 

1.  Potassium  dichromate 5.5  grams 

Water,  to 100  cubic  centimeters 

2.  Hydrochloric  acid  (concen- 

trated)         5.5  cubic  centimeters 

Water,  to 100  cubic  centimeters 

For  use  take  one  part  of  Number  i ,  one  part  of  Num- 
ber 2,  and  two  parts  of  water.  This  mixture  does  not 
keep  well,  and  should  be  made  only  as  it  is  to  be  used.  If 
the  plate  is  dry,  it  must  be  soaked  in  water  for  an  hour. 
Leave  it  in  the  chromium  solution  until  bleaching  is 
completed,  wash  it  free  from  all  chromic  acid,  expose 
it  to  daylight  for  a  few  minutes,  and  then  redevelop  it, 
best  in  a  developing  solution  suited  to  bromide  paper. 
Finally  wash  thoroughly.  By  this  process  the  printing 
value  of  a  negative  may  be  increased  to  one  and  one- 
half  times  its  original  printing  value. 

When  a  dichromate  solution  is  acidified,  chromic 
acid  is  liberated,  an  unstable  compound  which  is  a 
good  oxidizer.  With  hydrochloric  acid  in  concentrated 
solution  chromic  acid  interacts,  producing  chlorine 
and,  by  the  consequent  reduction  of  the  chromic  acid, 
chromic  chloride.  Thus,  even  in  dilute  solution,  there 
is  a  tendency  for  this  reaction  to  go  on,  which  explains 
the  poor  keeping  qualities  of  the  mixed  solution.  The 
action  of  the  intensifying  solution  upon  the  plate  may, 


AFTER-TREATMENT  OF  NEGATIVE     103 

therefore,  be  considered  to  consist  first  in  an  oxidation 
of  silver  to  silver  oxide.  This,  in  presence  of  hydro- 
chloric and  chromic  acids,  will  be,  at  least  superficially, 
converted  to  silver  chloride  and  silver  chromate,  since 
both  are  insoluble  substances.  But  in  the  oxidation  of 
the  silver  an  equivalent  amount  of  chromic  acid  is 
reduced,  possibly  forming  an  approximation  to  chromic 
oxide  because  of  the  presence  of  an  abundance  of  water 
and  the  small  concentration  of  the  hydrochloric  acid. 
The  exposure  to  light  reduces  the  silver  salts  to  sub- 
salts,  after  which  the  reducing  agent  of  the  developer 
causes  the  redeposition  of  elementary  silver.  But  along 
with  this  action  there  has  been  formed  a  corresponding 
quantity  of  chromic  oxide,  more  or  less  hydrated,  and 
quite  insoluble,  the  addition  of  which  to  the  original 
silver  causes  a  considerable  increase  in  the  opacity  of 
the  negative. 

Occasionally,  even  the  most  expert  photographer  will 
find  himself  possessed  of  a  negative  which  is  altogether 
too  dense  throughout.  The  matter  may  be  remedied  by 
submitting  the  plate  to  the  process  which,  photographic- 
ally, is  known  as  "reduction,"  but  which  consists, 
chemically,  simply  in  the  oxidation  of  some  of  the  silver 
of  the  image  and  the  conversion  of  the  oxidized  silver 
to  a  soluble  compound,  which  is  then  removed  in 
solution.  There  is  a  considerable  number  of  oxidizing 
agents  which  can  be  utilized  for  this  purpose.  It  has 
been  found  possible  so  to  regulate  the  action  of  some  of 
these  as  to  produce  somewhat  different  results  with  the 
different  oxidizers.  In  the  case  just  mentioned,  where 
there  is  a  general  opacity  all  over  the  plate,  it  may  be 
desirable  to  diminish  this  opaqueness  with  a  fair 
degree  of  uniformity  throughout  the  scale  of  gradation. 


104    CHEMISTRY  FOR  PHOTOGRAPHERS 

If  so,  the  "permanganate  reducer"  may  be  employed, 
since  it  gives  a  nearly  uniform  reduction,  acting  only 
a  little  more  strongly  upon  the  highlights  (i.e.  the 
densest  deposits  of  silver)  than  upon  the  shadow 
details.  To  prepare  the  solution,  dissolve  0.5  gram  of 
potassium  permanganate  in  200  cubic  centimeters  of 
water  and  add  ^  cubic  centimeters  of  concentrated 
sulphuric  acid.  This  stock  solution  is  four  times  too 
concentrated,  so  that  in  using  it  for  the  reduction  of  a 
plate  one  part  of  the  permanganate  is  to  be  diluted  with 
three  parts  of  water.  The  negative,  if  dry,  is  well 
soaked  and  then  immersed  in  the  solution  until  suffi- 
ciently reduced  in  density.  The  dish  must  be  con- 
stantly rocked  to  secure  uniformity  of  action.  The  plate 
is  next  quickly  rinsed  in  two  or  three  changes  of  water 
and  transferred  for  five  minutes  to  a  previously  unused 
acid  thiosulphate  fixing  bath,  and  finally  washed  and 
dried.  Reaction: 

Silver  -f-  Potassium  permanganate  +  Sulphuric  acid 

=  Silver  sulphate  +  Potassium  sulphate 
+  Manganese  sulphate  +  Water 

The  products  of  this  reaction  are  all  soluble  in  water 
and  therefore  removable  from  the  gelatine  by  washing. 
The  function  of  the  thiosulphate  bath  is  to  stop  in- 
stantly the  action  of  the  small  amount  of  acidified 
permanganate  which  is  retained  by  the  gelatine  after 
the  plate  has  been  rinsed,  since  it  would  continue  to 
dissolve  away  a  little  more  silver.  Reaction: 

Sodium  thiosulphate  +  Potassium  permanganate 

+  Sulphuric  acid 

=  Sodium  sulphate  +  Potassium  sulphate 
+  Manganese  sulphate  +  Water 

A  reduction  method  which  is  very  useful  for  over- 


AFTER-TREATMENT  OF  NEGATIVE     105 

exposed,  or  foggy,  or  veiled,  negatives  is  the  one  which 
for  a  long  time  has  been  known  as  "Farmer's  reducer." 
The  solution  consists  of  a  mixture  of  ten  per  cent, 
potassium  ferricyanide  with  ten  per  cent,  "hypo,"  and 
acts  more  energetically  upon  the  details  in  the  shadows 
than  upon  the  highlights.  The  result  is  therefore  an 
increase  of  contrast,  desirable  for  the  class  of  negatives 
noted.  This  reducing  bath,  which  must  be  made  up 
just  before  it  is  needed,  can  be  conveniently  prepared 
by  dissolving  i  o  grams  of  sodium  thiosulphate  in  water 
and  diluting  to  100  cubic  centimeters,  and  lastly 
adding  5  to  10  cubic  centimeters  of  a  ten  per  cent, 
ferricyanide  solution.  Evenness  of  action  is  secured,  as 
usual,  by  rocking  the  tray  in  which  the  operation  is 
conducted.  Wash  the  plate  well  afterwards.  Reaction: 

Silver  -f  Potassium  ferricyanide  +  Sodium  thiosulphate 
=  Silver-sodium  thiosulphate  +  Potassium-sodium 

ferrocyanide 

Two  excellent  processes,  whereby  the  contrast  of  a 
plate  may  be  diminished,  are  found  in  the  eerie  sulphate 
and  the  ammonium  persulphate  reducers,  the  latter 
being  known  as  "Bennett's  reducer."  In  the  former, 
advantage  is  taken  of  the  powerful  oxidizing  action  of 
the  cerium  compounds  in  which  cerium  is  in  the 
higher  condition  of  oxidation,  having  present  also  an 
acid,  such  as  sulphuric  acid,  with  which  the  oxidized 
silver  forms  a  soluble  product.  A  stock  bath  may  be 
made  up  by  dissolving  10  grams  of  eerie  sulphate 
crystals  in  water,  acidifying  with  4  cubic  centimeters 
of  concentrated  sulphuric  acid,  and  diluting  this 
mixture  with  water  to  100  cubic  centimeters.  Take  for 
use  one  part  of  the  stock  solution  and  one  part  of  water. 
It  will  be  found  to  attack  the  highlights  more  strongly 


106    CHEMISTRY  FOR  PHOTOGRAPHERS 

than  the  details  in  the  shadows.  After  reduction,  wash 
well  and  dry  the  plate.  Reaction : 

Silver  +  Ceric  sulphate  +  Sulphuric  acid 

=  Silver  sulphate  +  Cerous  sulphate  +  Water 

For  "Bennett's  reducer"  make  the  following  solution: 
Dissolve  in  water  1 2  grams  of  ammonium  persulphate, 
add  2  grams  of  sodium  sulphite  and  i  cubic  centimeter 
of  concentrated  sulphuric  acid,  and  dilute  to  i  oo  cubic 
centimeters.  Use  this  solution  in  the  proportion  of  one 
part  to  four  to  eight  parts  of  water,  and  apply  it  to  the 
wet  negative,  rocking  the  tray  while  the  action  is  going 
on.  Rinse  the  plate  quickly  and  immerse  it  for  six 
minutes  in  a  ten  per  cent,  acid  "hypo"  fixing  bath, 
which  will  stop  the  action  in  the  same  way  as  with  the 
permanganate  reducer.  In  order  to  get  the  full  advan- 
tage of  this  method  in  lessening  the  contrasts  of  a  neg- 
ative, it  is  important  not  to  allow  the  persulphate  to 
act  too  long  upon  the  plate.  For  a  limited  time  the  so- 
lution will  dissolve  away  the  densest  portions  of  the  de- 
posited silver  without  appreciably  attacking  either  the 
middle  tones  or  the  finest  details  in  the  shadows.  But 
if  the  plate  is  left  for  a  longer  time  in  the  bath,  the 
shadow  details  will  be  materially  affected.  Therefore, 
it  is  necessary  to  watch  the  operation  closely,  removing 
the  negative  from  the  persulphate  solution  the  instant 
that  a  solvent  effect  appears  in  the  thinner  parts. 
Reaction : 

Silver  +  Ammonium  persulphate 

=  Silver  sulphate  +  Ammonium  sulphate 

It  is,  perhaps,  more  frequently  the  case  that  both 
these  kinds  of  processes,  of  intensification  and  reduction, 
will  be  utilized,  than  that  either  one  of  them  will  be 
applied  alone.  For  example,  when  the  range  of  contrast 


AFTER-TREATMENT  OF  NEGATIVE     107 

of  a  given  subject  that  must  be  photographed  is  too 
great  to  be  expressed  by  the  emulsion,  it  is  quite 
feasible  to  adjust  the  exposure  and  the  development  to 
the  shadow  details,  and,  after  fixing  and  washing  the 
plate,  first  reduce  the  highlights  with  persulphate  and 
then,  if  required,  intensify  the  whole  negative,  as  with 
the  uranium  bath.  By  this  means  a  pleasing  picture 
can  oftentimes  be  made,  under  unfavorable  conditions 
of  lighting,  showing  the  subject  which  is  photographed 
as  it  appears  when  advantageously  lighted.  These 
purely  chemical  methods  of  modifying  the  negative 
will  be  found  among  the  most  useful  helps  to  the 
photographer  in  saving  numbers  of  negatives  which 
would  otherwise  be  wasted. 


CHAPTER  VIII 
PRINTING  PROCESSES  WITH  SILVER  SALTS 


THE  object  of  all  the  solicitude  which  has  been 
expended  in  exposing  the  plate,  in  carrying  out  its 
development,  and  in  any  subsequent  treatment  applied 
to  it  is  merely  to  provide  a  means  for  expressing  in  the 
final  print  the  varied  tones  of  light  and  shade  which, 
photographically  speaking,  constitute  the  subject 
portrayed.  Whether  it  be  the  photographer's  purpose 
to  make  a  picture  which  shall  record  facts  as  ac- 
curately as  possible  or  whether  it  be  his  intention  to 
represent  a  mood  or  a  phase  of  the  subject,  giving 
expression  in  the  print  to  his  own  artistic  feeling,  the 
quality  of  the  negative  determines  very  largely  the 
quality  of  the  finished  picture.  Successful  photography 
depends  in  the  first  instance  upon  the  worker's  ability 
to  make  properly  exposed  and  developed  negatives. 
But  in  the  wide  range  of  printing  media  at  his  com- 
mand, and  especially  in  view  of  the  variety  of  qualities 
and  surfaces  in  which  the  popular  brands  of  papers 
may  be  obtained,  there  is  the  possibility  of  a  consider- 
able degree  of  control.  It  becomes,  therefore,  a  matter 
of  no  small  importance  to  select  a  medium  which  is 
suitable  for  the  purpose  in  hand,  whatever  it  may  be, 
and  to  determine  the  precise  character  of  negative 
best  adapted  for  printing  in  that  medium.  All  printing 
processes  depend,  for  their  effect,  upon  the  use  of  a 
substratum  or  support,  which  is,  nearly  universally, 
white  paper,  or  paper  that  is  at  most  but  slightly 
tinted.  The  highest  light  is  thus  the  plain,  unshaded 
1 08 


PRINTING  WITH  SILVER  IOQ 

paper,  the  deepest  shadow  is  formed  by  deposits  heavy 
enough  to  obscure  the  paper  entirely,  and  the  inter- 
mediate tones  are  produced  by  the  different  densities 
of  deposit,  which,  veiling  the  substratum  to  different 
degrees,  allow  the  transmission  of  more  or  less  light  by 
the  deposit  and  its  reflection  by  the  substratum.  Since 
not  only  are  most  of  the  printing  processes  which 
depend  upon  silver  salts  the  simplest  to  use,  but  also, 
largely  for  that  reason,  of  course,  are  they  undoubtedly 
the  most  extensively  employed,  we  shall  begin  with 
them  our  discussion  of  the  different  methods  of  making 
prints. 

The  action  of  light  upon  silver  compounds,  and  in 
particular   its   effect   upon   the   silver   halogen   salts, 
whereby,  according  to  the  reaction : 
Silver  chloride  +  Light 

=  Silver  sub-chloride  +  Chlorine, 

the  so-called  latent  image  is  formed,  has  already  been 
sufficiently  described  in  previous  chapters.  But  in  the 
case  of  photographic  papers,  as  compared  with  plates  or 
films,  we  must  note  that  there  are  certain  differences 
produced  by  the  use  of  the  different  kinds  of  support. 
In  the  plates  and  films  we  find  a  comparatively  thick 
layer  of  photographic  emulsion  spread  upon  glass  and 
celluloid,  respectively,  both  these  substrata  possessing 
no  appreciable  chemical  activity  towards  the  emulsion. 
Paper,  however,  is  an  organic  substance,  which,  con- 
sisting largely  of  cellulose,  is  readily  capable  of  inter- 
acting chemically,  especially  with  silver  compounds. 
It  is,  moreover,  necessary,  in  order  that  the  deposited 
silver  should  be  kept  strictly  upon  the  surface,  that  the 
paper  support  be  thoroughly  sized.  And  this  sizing, 
generally  arrowroot,  starch,  or  gelatine,  is  also  organic 


no    CHEMISTRY  FOR  PHOTOGRAPHERS 

and  chemically  active  in  the  same  way  as  the  paper. 
In  the  sunlight  printing  paper,  P.O.P.,  as  it  is  called, 
there  is  the  further  difference  that  the  light  is  allowed 
to  produce  a  visible  effect,  whereupon,  instead  of  its 
being  treated  with  a  reducing  agent,  the  image  is 
acted  upon  by  metallic  salts.  By  these,  in  conjunction 
with  the  fixing  bath,  a  part  of  the  silver  salt  is  removed 
and  the  final  image  consists  of  silver  and  some  other 
metal  or  metallic  compound  added  to  it.  For  making 
this  printing-out  paper  there  is  required  a  binding 
material  in  which  the  photo-sensitive  silver  salt  may 
be  formed.  Albumen,  collodion,  and  gelatine  are  three 
such  binders  which  have  been  much  used,  the  last 
probably  being  most  employed  at  the  present  time.  A 
salt  bath  is  prepared  containing  soluble  chloride  and  the 
binder,  and  the  sheets  of  paper  are  given  a  coating, 
which  can  very  simply  be  done  by  floating  the  paper 
upon  the  solution.  After  it  has  been  dried,  the  "plain 
salted  paper"  is  again  floated,  upon  a  silver  nitrate 
solution  of  suitable  concentration,  and  dried  in  non- 
actinic  light.  It  is  then  ready  for  use,  but  thus  pre- 
pared has  the  disadvantage  of  not  keeping  well  for 
any  considerable  length  of  time.  For  the  salt  bath, 
make  up  a  solution  consisting  of: 

Ammonium  chloride 6  to  8  grams 

Sodium  citrate . 10  grams 

Sodium  chloride 2  to  3  grams 

Gelatine i  gram 

Distilled  water,  to 440  cubic  centimeters 

First,  dissolve  the  gelatine  in  hot  water  and  then  add 
the  other  compounds  and  filter  the  resulting  solution. 
Pour  the  solution  into  a  tray  of  sufficient  size.  Grasp 
a  sheet  of  the  paper  to  be  salted  by  two  diagonally 
opposite  corners  and  bend  the  sheet  upon  the  other 


PRINTING  WITH  SILVER  1 1 1 

diagonal  as  an  axis,  by  bringing  the  hands  together. 
Lower  the  paper  until  it  touches  the  surface  of  the  liquid 
along  the  axis,  and  then  by  separating  and  lowering  the 
hands  allow  the  full  area  of  the  sheet  to  lie,  without  any 
air  bubbles,  upon  the  solution.  Float  the  paper  thus 
for  three  minutes,  after  which  it  is  to  be  removed  slowly 
and  hung  up  by  one  corner  to  dry.  It  is  then  ready  to  be 
sensitized  by  similar  flotation  upon  a  solution  of  silver 
nitrate  in  the  proportion  of  one  part  of  the  solid  to 
nine  parts  of  water.  In  hot  weather  the  salted  paper 
may  be  floated  for  three  minutes,  but  in  cold,  for  five 
minutes.  Again  hang  up  the  sheet  by  one  corner  to 
dry,  but  as  it  is  now  light-sensitive,  drying  must  take 
place  away  from  actinic  light.  The  keeping  qualities 
of  this  paper,  as  previously  stated,  are  poor,  as  it  will 
be  good  for  about  two  days  in  warm,  but  for  a  week  in 
cold,  weather.  The  paper,  however,  can  be  "salted" 
in  quantity  and  sensitized  only  as  desired. 

It  is  now  in  order  to  consider  the  changes  which  occur 
in  the  printing,  toning,  and  fixing  of  such  paper.  First 
it  will  be  apparent  that  there  is  present  in  the  very  thin 
coating  of  the  unprinted  paper  a  considerable  number 
of  substances,  since,  from  the  nature  of  the  operations 
just  described,  it  is  impossible  for  all  of  even  the 
soluble  products  of  the  reactions  to  escape.  When  the 
salted  paper  is  sensitized  upon  the  silver  nitrate,  the 
following  reactions  must  occur : 

1 i )  Ammonium  chloride  -f  Silver  nitrate 

=  Silver  chloride  +  Ammonium  nitrate 

(2)  Sodium  chloride  +  Silver  nitrate 

=  Silver  chloride  +  Sodium  nitrate 

(3)  Sodium  citrate  +  Silver  nitrate 

=  Silver  citrate  +  Sodium  nitrate 

(4)  Organic  compounds  in  sizing  and  paper  +  Silver  nitrate 

=  Complex  organo-silver  compound  +  Nitric  acid 


ii2    CHEMISTRY  FOR  PHOTOGRAPHERS 

All  of  these  insoluble  silver  compounds  and  greater 
or  less  amounts  of  the  soluble  products  of  the  reactions, 
besides  a  portion  of  the  silver  nitrate  from  the  sensitiz- 
ing bath,  will  be  present  in  the  coating  and  soaked  up 
in  the  body  of  the  paper  support.  The  silver-organic 
compound  is  an  exceedingly  complex  substance  whose 
nature  is  not  at  all  well  understood.  It  is  said  to  be 
somewhat  less  easily  formed,  that  is,  is  rather  more 
soluble,  than  silver  chloride,  and  therefore  the  sensitized 
paper  will  in  general  have  more  of  the  chloride  than  of 
the  organic  compound  formed  upon  it,  the  relative 
proportions  of  the  two  depending  directly  upon  the 
concentrations  of  silver  nitrate  in  the  sensitizing  bath 
and  chloride  in  the  salted  paper.  If  the  sensitizing 
solution  is,  for  example,  sufficiently  dilute,  practically 
none  of  the  organic  silver  compound  will  form,  since  all 
the  silver  from  the  solution  in  contact  with  the  paper  will 
go  to  form  chloride.  This  organic  compound  is,  before 
exposure  to  light,  decomposed  by  sodium  thiosulphate, 
and  by  the  action  of  light  is  reduced  to  an  organic 
silver  oxide  possessing  a  reddish  tint.  When  the  photo- 
chemical reaction: 

Silver  chloride  +  Light 

=  Silver  sub-chloride  +  Chlorine 

takes  place  on  exposure  of  the  paper  to  light,  the 
liberated  chlorine,  by  a  secondary  reaction,  forms  with 
the  excess  of  silver  nitrate  present  more  silver  chloride : 

Silver  nitrate  +  Chlorine  +  Organic  matter 

=  Silver  chloride  +  Nitrated  organic  matter 

This  new  silver  chloride  is  also  photo-sensitive,  and 
thus  the  quantity  of  sub-chloride  is  increased.  If  the 
printed  paper  is  at  once  put  into  sodium  thiosulphate 
solution  to  remove  unreduced  silver  salt,  as  must  ulti- 


PRINTING  WITH  SILVER  113 

mately  be  done  to  get  a  permanent  print,  the  silver 
chloride  remaining  is  rendered  soluble  as  silver-sodium 
thiosulphate,  and  the  silver  sub-chloride  is  so  acted  upon 
as  to  form  this  same  soluble  compound  and  elementary 
silver.  The  reddish  reduction  product  from  the  complex 
silver-organic  substance  is  practically  unaffected  by 
thiosulphate.  Reactions : 

1 i )  Silver  chloride  +  Sodium  thiosulphate 

=  Silver-sodium  thiosulphate  +  Sodium  chloride 

(2)  Silver  sub-chloride  +  Sodium  thiosulphate 

=  Silver  +  Silver-sodium  thiosulphate  +  Sodium 

chloride 

So  the  image  left  after  fixing  and  washing,  of  a  rather 
unpleasant  red  color,  consists  of  metallic  silver  and 
complex  silver-organic  compounds.  Papers  sized  with 
gelatine  give  redder  tones  than  albumen  papers,  and 
those  sized  with  starch,  bluer  tones.  If  the  prints  on 
these  papers  are  merely  fixed  after  being  exposed  to 
the  action  of  light,  therefore,  the  resulting  pictures, 
being  very  unattractive,  are  totally  unsatisfactory, 
except  for  use  as  proofs.  By  treating  them,  before  fixing 
in  thiosulphate,  with  a  suitable  solution  of  gold  chloride, 
the  resultant  color  of  the  mixture  of  silver  and  gold  may 
be  made  very  pleasing.  If  a  gold  tri-chloride  solution 
be  used  upon  paper  from  which  the  excess  of  silver 
nitrate  has  been  washed  before  toning,  the  reduction 
of  gold  is  accompanied  by  a  simultaneous  oxidation  of 
the  sub-chloride  of  silver,  and  the  latter  compound  is 
finally  removed  as  soluble  thiosulphate.  In  this  way 
one  atom  of  gold  will  replace  three  atoms  of  silver,  and 
the  outcome  of  the  reaction  will  be  a  feeble  blue  deposit 
of  gold,  as 

3  Silver  sub-chloride  +  Gold  trichloride 

=  Gold  +  3  Silver  chloride 


ii4    CHEMISTRY  FOR  PHOTOGRAPHERS 

But  if  the  gold  solution  is  so  regulated  that  the  tri- 
chloride is  reduced  to  monochloride,  that  is,  auric 
chloride  is  converted  by  reduction  to  aurous  chloride, 
then,  in  the  reaction  between  the  aurous  chloride  of  the 
toning  bath  and  the  silver  sub-chloride  of  the  print, 
there  will  be  a  replacement,  atom  for  atom,  of  gold  for 
silver,  and  a  stronger  image  will  result.  Reaction : 

Silver  sub-chloride  +  Aurous  chloride 

=  Gold  +  Silver  chloride 

In  making  up  gold  toning  solutions  auric  chloride  is 
the  form  of  gold  compound  used,  but  it  is  necessary 
either  to  use  hot  water  for  dissolving  the  salts,  and  then 
wait  for  the  solutions  to  become  cool,  or  to  allow  the 
cold  solutions  to  stand  for  a  day  or  two  before  using 
them.  In  this  manner  the  auric  salt  is  partially  reduced 
to  aurous  chloride  in  the  solution,  after  which  action 
the  toning  baths  work  properly.  Such  a  toning  bath 
is  the  following  recommended  by  Abney: 

Gold  trichloride i  part 

Calcium  hypochlorite  ("Chloride  of  lime")  i  part 

Calcium  carbonate  ("Chalk") i  teaspoonful 

Water 3  500  parts 

If  this  bath  is  made  up  with  hot  water  it  can  be  used 
as  soon  as  it  has  cooled.  Gold  trichloride  as  usually 
prepared  contains  an  excess  of  free  hydrochloric  acid, 
and  it  is  partly  for  .the  purpose  of  neutralizing  the 
acid  that  the  calcium  carbonate  is  added  to  the  bath. 
Reaction : 

Calcium  carbonate  +  Hydrochloric  acid 

=  Calcium  chloride  -f  Carbon  dioxide  +  Water 

Some  salts  have  the  property  of  interacting  to  a 
greater  or  less  extent  with  water  in  which  they  are  dis- 
solved, the  reaction  being  called  "hydrolysis."  It  is  prac- 


PRINTING  WITH  SILVER  1 1 5 

tically  the  reverse  of  a  reaction  of  neutralization  in 
which  base  and  acid  unite  to  form  a  salt  and  water, 
since  in  hydrolytic  action  a  salt  and  water  interact  to 
form  a  base  and  an  acid.  Calcium  hypochlorite  hydro- 
lyzes  in  this  manner  to  some  degree,  and  hypochlorous 
acid  and  calcium  hydroxide  tend  to  form  in  the  solution. 
Likewise,  when  auric  chloride  is  contained  in  the  same 
solution  with  the  calcium  hydroxide,  it  may  be  con- 
sidered to  react  with  the  latter  to  produce  aurous 
chloride,  calcium  hypochlorite,  and  water,  thus: 

Auric  chloride  +  Calcium  hydroxide 

=  Aurous  chloride  +  Calcium  hypochlorite 

+  Water 

A  similar  reaction  may  occur  with  the  solid  calcium 
carbonate  placed  in  the  toning  bath : 

Auric  chloride  +  Calcium  carbonate 

=  Aurous  chloride  +  Calcium  hypochlorite 
+  Carbon  dioxide 

The  gold  salt  in  such  a  solution  may  be  said  to  be 
chemically  in  a  state  of  neutral  equilibrium;  the  least 
disturbance  renders  the  equilibrium  unstable,  and 
metallic  gold  begins  to  precipitate.  If  into  this  bath 
there  is  put  a  print,  owing  to  the  presence  of  the  sub- 
chloride  of  silver  and  the  light-reduced,  silver-organic 
oxide,  the  chemical  balance  of  the  solution  is  upset, 
gold  commences  to  separate,  and  the  sub-compounds 
of  silver  are,  by  chlorine  from  the  aurous  chloride, 
oxidized  back  to  silver  chloride,  according  to  the  re- 
action previously  written.  Before  toning  with  the  gold 
solution  it  is  usual  to  wash  the  printed  paper  in  several 
changes  of  water  in  order  to  free  it  as  much  as  possible 
from  silver  nitrate.  Any  soluble  silver  salt  not  washed 
out  of  the  paper  is  at  once  precipitated  in  the  toning 


u6    CHEMISTRY  FOR  PHOTOGRAPHERS 

bath  as  silver  chloride  by  action  of  the  gold  chloride. 
The  hypochlorite  acts  as  a  restrainer  in  the  bath,  re- 
tarding the  reduction  of  gold.  Perhaps  it  does  this  in 
two  ways,  since  not  only  does  hypochlorous  acid  de- 
compose spontaneously,  forming  hydrochloric  acid  and 
oxygen,  by  the  reaction: 

Hypochlorous  acid  =  Hydrochloric  acid  +  Oxygen 

but  it  also  reacts  with  hydrochloric  acid  directly, 
liberating  chlorine : 

Hypochlorous  acid  +  Hydrochloric  acid 

=  Chlorine  +  Water 

The  oxygen  evolved  must  tend  to  reoxidize  the  gold, 
thus  diminishing  its  rate  of  reduction;  and  the  free 
chlorine  also  tends  to  retard  this  reduction  by  oxidizing 
the  sub-chloride  of  silver  directly.  Evidently,  from  these 
considerations,  the  bath  should  be  expected  to  become 
acid  in  time,  and  this  is  in  fact  the  case. 

Another  toning  solution  may  be  prepared  from 
sodium  acetate  and  gold  trichloride,  but  it  must  be 
made  up  the  day  before  it  is  to  be  used,  to  allow  the 
partial  reduction  of  auric  to  aurous  chloride  to  occur. 
Afterwards  it  keeps  indefinitely,  and,  like  the  preceding 
bath,  it  may  be  strengthened  by  the  addition  of  more 
gold  trichloride  as  it  becomes  worked  out. 

Sodium  acetate 3o  parts 

Gold  trichloride i  part 

Water 4,400  parts 

This  formula  also  is  given  by  Abney.  Aurous  chloride 
may  be  formed  by  the  following  reaction,  in  the  ripening 
of  the  bath: 

Auric  chloride  +  Sodium  acetate 

=  Aurous  chloride  +  Sodium  trichloracetate 
+  Hydrochloric  acid 


PRINTING  WITH  SILVER  117 

In  using  the  solution  the  print  must  first  be  well 
washed  to  free  it  from  the  greater  portion  of  silver 
nitrate.  This  bath  also  becomes  after  a  time  too  acid 
to  work  well.  The  reaction,  by  which  metallic  gold  is 
substituted  for  silver,  is  the  same  as  previously  given : 

Silver  sub-chloride  +  Aurous  chloride 

=  Gold  +  Silver  chloride 

The  unfixed  image,  therefore,  after  toning,  is  com- 
posed of  metallic  gold  and  the  chloride  of  silver,  to- 
gether with  any  sub-chloride  of  the  latter  metal  which 
has  not  suffered  oxidation. 

To  fix  prints  after  toning,  they  should  be  well  washed 
in  water  and  then  put  for  ten  to  fifteen  minutes  into 
a  plain  solution  of  sodium  thiosulphate,  one  part,  to 
water,  five  parts;  after  this  they  may  be  soaked  for 
fifteen  minutes  and  finally  washed  in  running  water 
for  twelve  hours.  Reaction  : 

Silver  chloride  +  Silver  sub-chloride 

+  Sodium  thiosulphate 
=  Silver  +  Silver-sodium  thiosulphate 
+  Sodium  chloride 

The  tones  of  the  print  are,  in  part,  caused  by  the  vari- 
ous sizes  of  the  particles  of  gold  deposited,  the  color  by 
reflected  light  of  finely  divided  gold  ranging  from  purple 
to  a  dirty  green,  according  to  the  fineness  of  the  sub- 
division. An  over-toned  print  is  blue  on  account  of  the 
excess  of  gold  deposited.  The  final  tone  of  the  fixed 
print  is  due  to  the  mixture  of  colors,  by  reflected  light, 
of  the  silver  and  gold  of  which  the  image  is  composed. 
Before  fixing,  the  print  can  be  further  washed  and  put 
into  a  solution  of  a  platinum  salt  in  which  the  gold 
will  be  partially  replaced  by  platinum,  in  a  similar 
way  as  the  gold  itself  was  plated  upon  the  silver. 


ii8    CHEMISTRY  FOR  PHOTOGRAPHERS 

Formulas  for  the  platinum  toning  of  printing  out  paper 
are  the  following: 

Potassium  chloroplatinite .  .  2  grams 

Phosphoric  acid  (Sp.G.  1.12)         28  cubic  centimeters 
Water  (distilled),  to 1000  cubic  centimeters 

After  toning,  the  print  is  to  be  washed  and  then  fixed 
in  thiosulphate  and  washed  as  usual.  Or: 

Potassium  chloroplatinite . .  0.25  gram 

Sodium  chloride 2.5    grams 

Citric  acid 2.5    grams 

Water  (distilled),  to 500  cubic  centimeters 

To  use  this  solution,  tone  first  in  an  ammonium  sul- 
phocyanide  bath,  wash,  and  put  into  the  platinum  bath, 
and  finish  as  usual.  The  sulphocyanide  bath  will  be 
given  presently. 

A  gelatine  emulsion,  originated  by  Abney,  which 
can  be  coated  upon  paper  for  prints  or  upon  glass  for 
transparencies,  is  prepared  in  much  the  same  way  as 
the  gelatino-bromide  emulsion  for  plates  : 

1.  Sodium  chloride 3.7  grams 

Potassium  citrate 1.8  grams 

Water,  to 50  cubic  centimeters 

2.  Silver  nitrate 1 1  grams 

Water,  to 50  cubic  centimeters 

3.  Gelatine 15  grams 

Water,  to 175  cubic  centimeters 

After  2  and  3  have  been  mixed,  i  is  to  be  added  in 
small  quantities  successively  with  shaking.  When  the 
emulsion  has  set,  squeeze  it  through  cloth  into  cold 
water,  and  allow  it  to  soak  for  ten  or  fifteen  minutes. 
Pour  off  the  water,  drain  the  emulsion,  and  dissolve 
it,  with  the  aid  of  heat,  with  15  cubic  centimeters 
of  alcohol.  Paper  is  best  coated  with  this  mixture  by 
flotation.  After  printing,  the  paper  can  be  either  fixed 


PRINTING  WITH  SILVER  1 1  q 

at  once,  which  will  give  a  brown  color,  or  it  can  be  toned 
in  a  bath  made  up  of  ammonium  sulphocyanide  and 
gold  trichloride  : 

Ammonium  sulphocyanide.  . .  7  grams 

Gold  trichloride o.\i  gram 

Water,  to 1000  cubic  centimeters 

The  gold  salt  and  ammonium  sulphocyanide  interact 
to  form  a  soluble  double  compound,  according  to  the 
equation : 

Gold  trichloride  +  Ammonium  sulphocyanide 

=  Ammonium-gold  sulphocyanide 
-f-  Ammonium  chloride 

Gold  is  reduced  from  the  double  salt  by  the  action  of 
silver  sub-chloride,  perhaps  as  follows : 

Silver  sub-chloride  +  Ammonium-gold  sulphocyanide 
=  Gold  -f-  Silver  chloride  -f-  Ammonium 
sulphocyanide  -f-  Silver  sulphocyanide 

The  toned  print  is  rinsed,  fixed  in  thiosulphate  solu- 
tion, and  washed.  Chloride  printing  out  papers  are  man- 
ufactured in  which  the  toning  salts  are  incorporated  in 
the  emulsion,  so  that  after  printing  it  is  only  required 
to  immerse  the  dry  print  in  a  thiosulphate  bath,  in 
which  the  toning  proceeds  simultaneously  with  the 
fixing.  These  are  the  "self-toning"  papers.  But  it  is  at 
least  sometimes  true  that  the  attempt  to  secure  too 
great  simplicity  in  photographic  manipulations  may 
lead  to  results  less  permanent  and  inferior  in  beauty. 

The  foregoing  criticism  does  not  necessarily  apply  to 
the  second  great  order  of  silver  papers,  namely,  the 
developing  out  papers.  For  certainly,  if  they  are  worked 
with  sufficient  care,  there  appears  to  be  no  reason  why 
they  should  not  be  as  permanent  as  a  print  in  silver 
can  be  made,  or,  allowing  for  the  difference  between  the 
paper  and  glass  supports,  why  they  may  not  be  practic- 


120    CHEMISTRY  FOR  PHOTOGRAPHERS 

ally  as  permanent  as  the  negative  itself.  The  chief 
cause  of  the  lack  of  permanency  in  silver  images  seems 
to  be  the  incomplete  removal  of  thiosulphate,  which 
in  the  course  of  time  breaks  down,  being  in  reality  a 
none  too  stable  substance,  and  liberates  sulphur.  This 
produces  a  yellowing  of  the  paper.  Besides  this,  if 
any  intensification  or  toning  processes  are  applied  to  an 
image  which  still  retains  thiosulphate,  there  will  in- 
evitably occur  in  time  complicated  reactions  by  the 
products  of  which  the  plate  or  print  will  be  stained  or 
bleached.  In  the  making  of  photographs,  by  whatever 
method,  there  is  no  "dodge"  nor  "trick"  which  can 
possibly  surpass  an  intelligent  grasp  of  the  chemical 
principles  involved,  if  coupled  with  a  strict  application 
of  good  laboratory  practice. 

The  developing  papers  are  subdivided  into  two 
classes,  those  in  which  silver  chloride  is  the  sensitive 
salt  and  those  in  which  silver  bromide  is  used.  In  the 
former,  or  "gaslight"  papers,  the  light  sensitiveness  is 
not  too  great  to  permit  of  handling  the  paper  in  the 
weaker  illumination  of  an  ordinary  room  lighted  by 
lamps,  so  that  the  darkroom  is  eliminated.  But  in  the 
case  of  the  bromide  paper  the  sensitiveness,  approxi- 
mately one  twentieth  of  that  of  the  photographic 
plate,  is  so  great  that  the  paper  must  be  carefully 
protected  from  white  light.  The  action  of  light  upon 
these  salts,  in  the  formation  of  the  latent  image,  and 
the  development  of  the  image  have  already  been  dis- 
cussed in  their  respective  chapters,  and  nothing  further 
need  be  added  here  upon  these  points.  The  principal 
precautions  to  be  taken  in  working  these  papers  are 
to  use  a  developing  agent  which  gives  a  good  black 
deposit  of  silver,  to  adjust  the  solution  so  that  staining 


PRINTING  WITH  SILVER  121 

is  prevented,  and  to  fix  thoroughly  and  in  a  fresh 
"hypo"  solution,  finally  freeing  the  paper  as  completely 
as  possible  from  all  removable  substances  by  sufficient 
washing. 

If  it  is  desired  to  alter  the  ordinary  black  deposit  of 
silver  to  some  color,  there  is  available  a  considerable 
number  of  processes  for  the  purpose.  The  change  most 
commonly  made  is  from  black  to  sepia,  and  this  is 
effected  by  transforming  the  silver  image  into  one 
composed  of  silver  sulphide.  There  are  two  methods  of 
doing  this,  as  shown  in  the  experiments. 

Experiment  3 1 .  Make  up  500  cubic  centimeters  of  a 
solution  containing  35  grams  of  sodium  thiosulphate 
and  7  grams  of  powdered  potassium  alum,  heating  the 
water  to  boiling  for  the  purpose.  Put  into  this  (un- 
filtered)  bath  a  well-washed  print  and  observe  the 
alteration  of  the  black  image  to  brown. 

In  the  process  the  thiosulphate  is  decomposed  and 
the  sulphur  combines  with  the  silver,  forming  silver 
sulphide.  The  slow  breaking  down  of  the  thiosulphate 
is  due  to  the  fact  that  aluminum  salts  hydrolyze, 
tending  to  go  over  to  aluminum  hydroxide,  thus 
liberating  acid  in  the  solution.  This  acid  causes  the 
decomposition  of  the  thiosulphate.  The  action  may 
be  represented  as  follows  by  equations  in  steps : 

1 .  Potassium  aluminum  sulphate  +  Water 

=  Potassium  aluminum  hydroxy-sulphate 
+  Sulphuric  acid 

2.  Sodium  thiosulphate  +  Sulphuric  acid 

=  Sodium  sulphate  +  Sulphur  +  Sulphur  dioxide 

3 .  Silver  +  Sulphur  =  Silver  sulphide 

A  better  way  of  changing  the  silver  to  silver  sulphide 
consists  in  first  oxidizing  the  image,  as  to  silver  bromide, 


iz2    CHEMISTRY  FOR  PHOTOGRAPHERS 

and  then  treating  it  with  a  solution  of  a  sulphide, 
whereby  silver  sulphide  is  formed  by  metathesis. 

Experiment  32.  Dissolve  5  grams  each  of  potassium 
ferricyanide  and  potassium  bromide  in  enough  water 
and  dilute  to  250  cubic  centimeters.  Put  a  dry  print 
into  this  solution  until  completely  bleached.  Rinse  the 
print  thoroughly  and  transfer  to  a  solution  of  sodium 
sulphide,  in  which  the  formation  of  silver  sulphide 
gives  the  sepia  color. 

The  stock  sulphide  solution  is  prepared  by  dis- 
solving 25  grams  of  pure  sodium  sulphide  in  250  cubic 
centimeters  of  water.  If  impure  sodium  sulphide  is 
used,  dissolve  in  150  cubic  centimeters  of  water,  boil, 
and  dilute  to  250  cubic  centimeters.  The  impurity  is 
precipitated  and  will  settle  to  the  bottom,  where  it 
may  be  left,  the  liquid  being  decanted  as  desired. 
For  use,  take  10  cubic  centimeters  of  the  stock  solution 
and  dilute  to  200.  Leave  the  print  in  this  diluted 
solution  until  it  is  certain  that  the  action  is  complete, 
wash  it  in  running  water  for  about  15  minutes,  and 
dry  it.  The  reactions  which  take  place  may  be  repre- 
sented as  follows: 

i.  Silver  +  Potassium  ferricyanide  +  Potassium 

bromide 
=  Silver  bromide  -f-  Potassium  ferrocyanide 

^.  Silver  bromide  +  Sodium  sulphide 

=  Silver  sulphide  +  Sodium  bromide 

Another  process  of  sulphide  toning,  by  which  it  is 
possible  to  control  the  depth  of  the  tone,  is  one  in 
which  mercuric  chloride  is  added  to  the  bleaching 
solution.  By  varying  the  proportions  of  the  mercury 
salt,  a  range  of  browns  from  sepia  to  a  pure  black  can 
be  secured.  This  process  is  naturally  not  quite  so  simple 


PRINTING  WITH  SILVER  123 

as  the  preceding.  Two  stock  solutions  which  keep  well 
are  required  for  bleaching: 

1 .  Potassium  ferricyanide ....         30  grams 
Potassium  bromide 45  grams 

Water,  to 250  cubic  centimeters 

2.  Mercuric  chloride 7  grams 

Potassium  bromide 7  grams 

Water,  to 250  cubic  centimeters 

The  prints  to  be  toned  should  each  have  been  fully 
developed  in  fresh  developer,  and  dried  after  fixing 
and  washing,  before  toning.  For  sepia,  take  i  part  of 
No.  i  and  dilute  with  1 1  parts  of  water.  One  part  of 
No.  i,  i  of  No.  2,  and  10  of  water  give  a  colder 
brown,  and  as  the  relative  proportion  of  the  second 
solution  is  increased,  the  tone  approaches  nearer  to 
black.  For  deep  brown,  take  ^  part  of  No.  i ,  i  part 
of  No.  2,  and  io><  parts  of  water;  brown-black,  i 
of  No.  i,  2  of  No.  2,  and  13  of  water;  and  for  a  pure 
black,  y-t  of  No.  i ,  2  of  No.  2,  and  q>£  parts  of  water. 
In  every  case  the  bleaching  should  be  thorough,  and 
if  any  of  No.  2  solution  is  used  in  the  bath  the  bleached 
prints  after  being  well  washed  must  be  transferred  for 
two  or  three  minutes  to  a  dilute  solution  of  hydro- 
chloric acid,  3  cubic  centimeters  of  the  concentrated 
acid  in  200  cubic  centimeters  of  water,  in  order  to 
prevent  the  mercury  salts  from  combining  with  the 
gelatine  coating.  Move  the  prints  on  into  a  second  and 
third  similar  acid  bath,  and  finally  wash  for  twenty 
minutes.  After  this  the  sulphide  toning  is  carried  out 
in  a  bath  made  by  diluting  to  200  cubic  centimeters 
15  cubic  centimeters  of  the  stock  solution  of  sodium 
sulphide  above  described.  The  diluted  sulphide  toning 
solution  should  be  used  but  once.  After  toning,  the 
print  is  to  be  thoroughly  washed,  say  for  a  half  hour, 


124    CHEMISTRY  FOR  PHOTOGRAPHERS 

and  then  dried.  Printing-out  paper  cannot  be  success- 
fully sulphide-toned.  The  reactions  may  be  outlined 
thus: 

i.  Silver  +  Potassium  ferricyanide 

+  Potassium  bromide 

=  Silver  bromide  +  Potassium  ferrocyanide 
^.  Silver  +  Mercuric  chloride 

=  Silver  chloride  +  Mercurous  chloride 

3.  Silver  bromide  +  Sodium  sulphide 

=  Silver  sulphide  -f  Sodium  bromide 

4.  Silver  chloride  +  Sodium  sulphide 

=  Silver  sulphide  +  Sodium  chloride 

5.  Mercurous  chloride  +  Sodium  sulphide 

=  Mercuric  sulphide  +  Mercury  +  Sodium  chloride 

The  toned  image  is  therefore  composed  of  silver  and 
mercuric  sulphides  with  elementary  mercury,  if  any 
of  the  No.  2  bleaching  solution  is  included  in  the  bath. 
The  character  of  the  tone  is  determined  by  the  pro- 
portion of  black  mercury  and  black  mercuric  sulphide 
added  to  the  brown  silver  sulphide  image. 

For  producing  a  variety  of  colors,  some  ranging 
from  a  warm  black  through  brown  to  red,  and  for  blues 
and  greens,  salts  of  copper,  uranium,  iron,  and 
vanadium  are  used.  So  far  as  concerns  the  permanency 
of  the  results,  it  is  well  to  observe  at  the  start  that  the 
pure  silver  image  and  the  image  in  silver  sulphide  may 
be  considered  sufficiently  durable  if  the  chemical  work 
is  well  done.  But  when  the  simple  metallic  silver  image 
is  converted  into  complexes  containing  silver  salts  with 
copper,  uranium,  iron,  or  vanadium  compounds  re- 
spectively, the  way  is  opened  not  only  for  the  gradual 
decomposition  of  these  complicated  bodies,  but  more 
especially  for  the  attack  upon  them  of  an  atmosphere 
containing  moisture  and  other  impurities.  Conse- 
quently the  experimenter  should,  knowing  the  dangers, 


PRINTING  WITH  SILVER  125 

pay  all  the  more  attention  to  cleanliness  in  manipulation 
and  in  particular  to  the  thoroughness  with  which  soluble 
products  are  washed  out  of  the  prints;  and  even  then 
should  not  be  unprepared  for  alterations  in  them  in  the 
lapse  of  time. 

It  will  be  most  convenient  to  make  up  a  number  of 
stock  solutions  from  which  the  different  toning  baths 
can  be  quickly  and  easily  prepared  whenever  wanted: 

SOLUTION  A 

Potassium  citrate 100  grams 

Water,  to 1000  cubic  centimeters 

SOLUTION  B 

Potassium  ferricyanide 25  grams 

Water,  to 250  cubic  centimeters 

SOLUTION  c 

Copper  sulphate 25  grams 

Water,  to 250  cubic  centimeters 

SOLUTION  D 

Uranium  nitrate 5  grams 

Water,  to 50  cubic  centimeters 

SOLUTION   E 

Ferric  ammonium  citrate 12.5  grams 

Water,  to 250  cubic  centimeters 

Toning  with  copper  gives  a  range  of  colors  from  a 
warm  black  through  brown,  purple,  many  shades  of 
red,  to  the  so-called  "red  chalk."  The  image  which  is 
finally  obtained  consists  of  a  mixture  of  silver  ferro- 
cyanide,  copper  ferrocyanide,  and  silver.  Take  of  the 
solutions : 

Solution  A ioo  cubic  centimeters 

Solution  B 10  cubic  centimeters 

Solution  C 10  cubic  centimeters 

and  place  the  wet  print  in  the  mixture  until  it  has 
altered  to  the  desired  color.  The  action  may  be  stopped 
at  any  point  by  washing  for  ten  minutes,  after  which 


n6    CHEMISTRY  FOR  PHOTOGRAPHERS 

the  print  is  to  be  dried.  The  following  reactions  may 
be  supposed : 

i .  Silver  +  Potassium  ferricyanide 

=  Silver  ferrocyanide  +  Potassium  ferrocyanide 
z.  Copper  sulphate  +  Potassium  ferrocyanide 

=  Copper  ferrocyanide  +  Potassium  sulphate 

To  get  red  tones  more  quickly,  an  alkaline  bath  may 
be  made  by  dissolving  2  grams  of  copper  sulphate  and  5 
grams  of  potassium  ferricyanide  in  1,000  cubic  centi- 
meters of  saturated  ammonium  carbonate  solution. 

By  referring  to  the  preceding  chapter,  it  will  be  re- 
called that  uranium  salt  is  used  in  a  process  for  in- 
tensifying the  silver  image.  For  this  reason  the  uranium 
toning  solution  is  not  adapted  to  dense  prints,  since 
they  are  intensified  by  it.  And,  further,  we  remember 
that  prolonged  washing  of  the  uranium-intensified 
image  results  in  weakening  it.  This  action  is,  of  course, 
more  pronounced  in  the  case  of  a  print,  and  therefore 
the  final  washing  cannot  be  too  long  continued  if  the 
print  is  to  hold  its  tone.  This  not  only  indicates  a 
quite  appreciable  solubility  of  the  uranium  ferrocya- 
nide, but  also  does  not  tend  towards  permanency  of  the 
image.  Take  of  the  stock  solutions  as  follows : 

Solution  B 10  cubic  centimeters 

Solution  D 5  cubic  centimeters 

Glacial  acetic  acid 10  cubic  centimeters 

Water 400  cubic  centimeters 

and,  after  the  tone  wished  for  is  reached,  wash  the  print 
in  two  or  three  changes  of  water  acidified  with  acetic 
acid  (one  part  acid  to  100  parts  of  water),  and  wash 
not  longer  than  1 5  minutes.  The  colors  given  by  this 
formula  run  from  black  through  brown  to  a  red  that 
has  more  yellow  in  it  than  the  reds  given  by  the  copper 
bath.  The  reactions  are  the  same  here  as  those  written 


PRINTING  WITH  SILVER  127 

for  uranium  intensification,  and  therefore  need  not  be 
repeated.  If  not  enough  acetic  acid  is  used,  permanent 
yellow  stains  will  appear. 

In  order  to  color  the  print  blue,  take: 

Water 300  cubic  centimeters 

Solution  B 30  cubic  centimeters 

Solution  E 30  cubic  centimeters 

Nitric  acid  (concentrated) ....  2  cubic  centimeters 

In  the  acidified  solution  the  oxidation  of  metallic  sil- 
ver is  accompanied  by  the  reduction  of  an  equivalent 
amount  of  ferricyanide  to  ferrocyanide,  when  not  only 
is  silver  ferrocyanide  formed,  but  there  is  produced,  also, 
by  interaction  of  the  ferric  salt  present  with  the  ferro- 
cyanide, ferric  ferrocyanide,  which,  as  we  saw  in  mak- 
ing the  tests  for  ferric  and  ferrous  iron,  is  the  substance 
known  as  "Prussian  blue."  It  is  quite  insoluble,  and 
therefore  adds  itself  to  the  image.  Too  long  washing 
after  the  print  has  been  toned  tends  to  bleach  it  out, 
since  the  blue  ferric  ferrocyanide  is  after  all  slightly 
soluble  in  water.  Reactions : 

i.  Silver  -f-  Potassium  ferricyanide 

=  Silver  ferrocyanide  -j-  Potassium  ferrocyanide 
i.  Ferric  ammonium  citrate  +  Potassium  ferrocyanide 
=  Ferric  ferrocyanide  +  Potassium  citrate 
+  Ammonium  citrate 

Green  tones,  which  become  lighter  with  longer  action 
of  the  bath,  can  be  secured  by  a  mixture  of  iron  and  vana- 
dium chlorides.  The  solution  is  a  rather  complicated  one : 

NO.   I 

Ferric  chloride 23  grams 

Oxalic  acid,  saturated  solution  125  cubic  centimeters 

Vanadium  chloride 4.6  grams 

Nitric  acid  (concentrated) ....  10  cubic  centimeters 

Water,  to 500  cubic  centimeters 

NO.  2 

Potassium  ferricyanide 23  grams 

Water,  to 500  cubic  centimeters 


128    CHEMISTRY  FOR  PHOTOGRAPHERS 

Add  No.  2  to  No.  i  with  constant  stirring  and  use  the 
mixture  without  further  dilution.  Prints  should  tone  in  i 
to  2  minutes,  and  are  then  to  be  fixed  in  a  solution  of: 

Sodium  thiosulphate 60  grams 

Boric  acid 12  grams 

Water,  to 300  cubic  centimeters 

After  fixing,  wash  for  10  minutes.  The  reduction  of 
the  vanadium  salt  may  be  accomplished  by  both  the 
finely  divided  silver  of  the  image  and  the  ferrocyanide 
produced  in  the  reduction  of  potassium  ferricyanide. 
Thus  vanadium  compounds  in  which  the  vanadium  is 
present  in  several  lower  stages  of  oxidation  are  probably 
added  to  the  image.  Mixtures  of  such  vanadium  com- 
pounds are  green.  If  the  reduction  of  vanadium  proceeds 
too  far,  as  by  allowing  the  print  to  remain  in  the 
bath  too  long,  the  resulting  color  is  lighter  owing  to 
changes  in  the  proportions  of  the  coloring  constituents. 

Bromide  paper  may  also  be  toned  with  a  platinum 
solution,  in  which  toning  for  about  20  minutes  gives  a 
good  black: 

Hydrochloric  acid  (concentrated)       i  cubic  centimeter 

Potassium  chloroplatinite i  gram 

Water  (distilled),  to 1000  cubic  centimeters 

The  prints  are  washed  thoroughly  after  toning,  to  get 
rid  of  the  acid,  are  fixed  in  sodium  thiosulphate  to  re- 
move the  silver  chloride  formed,  and  given  a  final 
washing.  Reactions: 

1 .  Silver  +  Potassium  chloroplatinite 

=  Platinum  +  Silver  chloride  +  Potassium  chloride 

2.  Silver  chloride  +  Sodium  thiosulphate 

=  Silver-sodium  thiosulphate 

Asimilar  formula  maybe  used  for  toning  lantern  slides : 

Hydrochloric  acid  (concentrated)  o .  2  cubic  centimeter 
Chloroplatinic  acid,  5%  solution       4  cubic  centimeters 
Water  (distilled),  to 1000  cubic  centimeters 


PRINTING  WITH  SILVER  129 

This  bath  gives  rapid  toning  with  some  reduction  in 
density.  The  reactions  are  similar  to  those  just  given. 

A  great  deal  of  study  has  been  put  by  the  manu- 
facturers of  photographic  materials  into  the  preparation 
of  ready-to-use  powders  and  solutions  for  performing 
practically  all  of  the  different  chemical  operations  in- 
volved in  the  making  of  photographs.  Doubtless  these 
same  manufacturers  have  spent  considerable  sums  of 
money  in  advertising  their  preparations.  While  it  is 
quite  true  that  these  may  be  on  occasion  extremely 
convenient  to  use,  nevertheless  it  is  equally  true  that 
it  is  both  more  costly  in  actual  cash  outlay  and  much 
less  enlightening  to  the  earnest  amateur  worker  to 
make  use  of  them.  It  would  be  idle  to  argue  that  the 
photographer  should  now,  as  he  was  once  obliged  to  do, 
make  his  own  plates,  when  they  can  be  so  much  better 
and  more  uniformly  turned  out  by  the  factories.  Some- 
thing can  be  said  on  both  sides  when  it  comes  to  the 
preparation  of  papers,  for,  as  we  shall  see  presently, 
there  are  several  interesting  and  beautiful  processes 
for  homemade  prints.  But  not  only  can  the  amateur 
save  a  little  money  by  purchasing  the  necessary 
chemicals  in  bulk  and  making  up  his  own  solutions,  he 
can,  if  he  will  allow  himself  to  become  interested, 
derive  a  great  deal  of  pleasure  and  satisfaction  from  the 
study  of  the  wonderful  chemical  processes  which  go 
to  make  up  this  fascinating  art  of  photography,  benefits 
that  are  largely  lost  to  those  who  work  only  with  ready- 
made  materials,  and  entirely  so  to  the  kind  that  "pushes 
the  button"  and  lets  the  professional  "do  the  rest." 


CHAPTER  IX 
PRINTING  PROCESSES  WITH  IRON  SALTS 


IN  an  earlier  chapter  reference  was  made  to  the  two 
classes  of  iron  salts,  ferric  and  ferrous,  and  several 
experiments  were  described  in  illustration  of  the  readi- 
ness with  which  they  can  be  reduced  and  oxidized 
respectively.  The  light-sensitiveness  of  ferric  com- 
pounds was  investigated  by  Sir  John  Herschel,  and  he 
indeed  invented,  in  1840,  one  of  the  simplest  photo- 
graphic printing  processes,  using  salts  of  iron,  namely, 
the  "blue  print"  process. 

Experiment  33.  Coat  a  piece  of  sized  paper  with  a 
little  ten  per  cent,  solution  of  ferric  ammonium  citrate, 
either  by  floating  the  paper  upon  the  liquid  or  by 
applying  the  solution  with  a  brush  or  a  wad  of  absorb- 
ent cotton.  When  the  coated  paper  has  become  dry, 
expose  it  for  one  or  two  minutes  to  sunlight  under  a 
negative.  Develop  the  print  in  a  little  dilute  potassium 
ferricyanide  solution  and  wash  it  thoroughly.  Note 
that  the  shadows  are  blue  and  the  highest  lights  white. 
Coat  another  piece  of  paper  with  the  same  ferric 
ammonium  citrate  solution,  expose  it  similarly  under  a 
positive  transparency,  such  as  an  unmounted  lantern 
slide,  and  develop  with  dilute  potassium  ferrocyanide 
and  wash.  Observe  in  this  case  also  that  a  blue  positive 
has  been  obtained,  but  from  a  positive  instead  of  from 
a  negative. 

By  the  action  of  light  the  ferric  citrate,  in  contact 
with  the  organic  matter  of  the  sizing,  has  been  reduced 

to  ferrous  citrate.  This  same  reaction  of  course  takes 
i  jo 


PRINTING  WITH  IRON  131 

place  in  both  parts  of  the  experiment.  Since  ferrous 
salts  interact  with  ferricyanide  to  form  an  insoluble 
blue  compound,  "Turnbull's  blue,"  the  application  of 
potassium  ferricyanide  to  the  first  print,  from  the 
negative,  forms  blue  ferrous  ferricyanide  wherever  any 
ferric  salt  has  been  reduced,  viz.,  in  the  shadows  and 
half  tones.  The  interaction  of  ferric  salts  with  ferri- 
cyanide produces  ferric  ferricyanide,  but  this  is  a 
soluble  compound,  and  therefore  washing  the  paper 
sufficiently  removes  all  the  unreduced  ferric  salt  and 
whatever  ferricyanide  is  not  combined  in  the  insoluble 
dark  blue  deposit.  Thus  a  blue  and  white  positive  is 
obtained  by  printing  under  a  negative.  Reactions : 

1 .  Ferric  ammonium  citrate  +  Light  +  Organic  matter 

=  Ferrous  ammonium  citrate  +  Organic 
oxidation  products 

2.  Ferrous  ammonium  citrate  -f-  Potassium  ferricyanide 

=  Ferrous  ferricyanide  +  Ammonium  citrate 
+  Potassium  citrate 

In  the  second  print,  from  the  positive,  the  unreduced 
ferric  compound  by  interaction  with  potassium  ferro- 
cyanide  forms  ferric  ferrocyanide,  an  insoluble  pre- 
cipitate of  dark  blue  "Prussian  blue,"  in  the  shadows. 
In  the  highlights  where  the  reduction  of  the  iron  salt  has 
occurred,  ferrous  citrate  reacts  with  the  ferrocyanide 
and  an  insoluble  precipitate  results,  which  is  originally 
ferrous  potassium  ferrocyanide  and,  like  most  ferrous 
salts,  white.  But  it  instantly  begins  to  oxidize,  absorb- 
ing oxygen  from  the  air,  and  rapidly  turns  blue  on 
account  of  the  oxidation  to  ferric  ferrocyanide.  Thus 
the  highlights  are  unavoidably  stained  a  light  blue  by 
the  mixture  of  white  ferrous  potassium  ferrocyanide 
with  dark  blue  ferric  ferrocyanide.  Reactions : 


1 32    CHEMISTRY  FOR^PHOTOGRAPHERS 

3 .  Ferric  ammonium  citrate  +  Potassium  ferrocyanide 

=  Ferric  ferrocyanide  +  Ammonium  citrate 
-f-  Potassium  citrate 

4.  Ferrous  ammonium  citrate  +  Potassium  ferrocyanide 

=  Ferrous  potassium  ferrocyanide  -f-  Ammonium 
citrate  +  Potassium  citrate 

5.  Ferrous  potassium  ferrocyanide  +  Oxygen  -f  Water 

=  Ferric  ferrocyanide  +  Potassium  ferrocyanide 
+  Potassium  hydroxide 

Blue  print  paper  as  commonly  prepared  is  coated  with 
a  mixture  of  ferric  ammonium  citrate  and  potassium 
ferricyanide  and  dried  in  the  dark.  It  does  not  keep 
well,  but  this  disadvantage  is  somewhat  counterbal- 
anced by  the  fact  that  it  is  so  easily  made.  When  it 
has  been  fully  exposed  to  the  action  of  light,  as  under  a 
negative  or  other  design,  it  requires  but  a  quarter  of  an 
hour's  washing  and  subsequent  drying  to  be  completed. 
As  soon  as  the  printed  paper  is  put  into  water,  the  re- 
duced ferrous  salt  and  the  potassium  ferricyanide  begin 
to  go  into  solution  and  instantly  react,  giving  the  in- 
soluble blue  ferrous  ferricyanide  wherever  the  light 
has  acted.  At  the  same  time  all  the  other  substances 
dissolve  and  are  washed  away,  and  the  print  is  there- 
fore developed  and  fixed  by  the  one  operation.  The 
reactions  are  the  same  as  in  (i)  and  (2)  above.  Washing 
must  not  be  too  long  continued,  since  the  blue  pre- 
cipitate has  an  appreciable  solubility  and  the  pro- 
longed application  of  water  degrades  the  image.  Water 
containing  lime  (i.e.,  hard  water)  has  a  greater  solvent 
effect  than  soft  water.  The  prints  can  be  made  some- 
what brighter  if  citric  acid  be  added  to  the  first  wash 
water,  about  2  grams  to  each  liter.  If  it  is  desired  to 
use  others  than  the  strictly  photographic  papers,  they 
should  be  sized.  This  may  be  done  by  mixing  1 5  grams 
of  arrowroot  to  a  paste  with  a  little  cold  water,  making 


PRINTING  WITH  IRON  133 

it  up  to  600  cubic  centimeters  with  water  and  boiling 
the  mixture  gently  until  it  becomes  clear.  The  thinner 
papers  may  be  immersed  in  sizing  them,  but  for  thick 
ones  it  is  better  to  brush  the  size  on  as  uniformly  as 
possible,  brushing  the  paper  one  way  and  then  across, 
and  finally  to  smooth  out  streaks  and  any  unevenness 
with  a  soft  sponge  or  tuft  of  absorbent  cotton.  The 
sheet  may  then  be  hung  up  with  clips  until  dry.  As  the 
sizing  has  a  tendency  to  drain  away  from  the  upper  edge 
and  accumulate  along  the  lower  edge  of  a  sheet  so  hung 
up,  it  is  sometimes  advisable  to  resize  the  paper  and  in 
drying  hang  it  the  other  side  up.  But  if  it  is  permitted 
to  become  nearly  or  quite  surface  dry  before  hanging  up, 
there  will  be  no  need  of  the  second  sizing.  For  coating 
the  paper  a  mixture  of  two  solutions  is  prepared : 

NO.   I 

Ferric  ammonium  citrate 160  grams 

Water,  to icoo  cubic  centimeters 

NO.  2 

Potassium  ferricyanide 1 20  grams 

Water,  to 1000  cubic  centimeters 

Fresh,  clean  crystals  of  the  ferricyanide  are  to  be  used. 
If  the  crystals  are  covered  with  a  powder,  they  should 
be  quickly  washed  in  a  little  water  before  making  the 
solution,  and  in  this  case  a  few  grams  more  than  the 
amount  specified  may  be  weighed  out.  These  solutions 
are  mixed  and  should  be  kept  in  a  brown  bottle  in  the 
dark.  The  mixture  will  keep  better,  and  for  a  longer 
time,  if  potassium  dichromate  be  added  in  the  propor- 
tion of  one  gram  per  liter.  When  ready  to  make  up 
some  paper,  filter  as  much  of  the  solution  as  may  be 
required  and  apply  it  evenly  with  a  brush  and  tuft 
of  absorbent  cotton.  The  coated  paper  should  be  dried 
as  quickly  as  possible  in  the  dark. 


134    CHEMISTRY  FOR  PHOTOGRAPHERS 

A  blue  print  process  which  is  useful  for  making 
positive  copies  from  positives  is  based  upon  the  second 
part  of  Experiment  33.  It  is  employed  particularly  for 
copying  tracings,  and  these  must  in  printing  be  put  into 
the  frame  with  the  back  in  contact  with  the  surface  of 
the  blue  print  paper.  The  coating  is  a  mixture  of  ferric 
salt  solution  with  a  little  gum  arabic.  After  printing, 
it  is  developed  in  a  mixture  of  ferrocyanide  and  ferri- 
cyanide,  in  which  both  the  reduced  and  the  unreduced 
iron  salts  form  insoluble  blue  precipitates,  but  where 
the  light  acted  the  gum  washes  off,  carrying  away  the 
blue  pigment  with  it,  so  that  the  image  consists  of 
a  blue  deposit  upon  a  white  ground. 

In  the  blue  print  processes  the  photographic  image 
is  formed  directly  from  the  iron  salt  itself.  Ferrous 
compounds,  as  we  remember,  are  excellent  reducing 
agents,  and  upon  this  fact  are  based  two  printing 
methods  which  are  indirect.  That  is,  the  image  is  de- 
posited, of  another  substance  from  that  reduced  by  the 
light,  by  a  secondary  reaction.  In  the  first  of  these, 
the  platinotype  process,  the  final  photographic  image 
is  formed  in  elementary  platinum  by  the  modification 
known  as  "platinum  black."  This  finely  divided  dense 
black  variety  of  the  element  is  conveniently  prepared 
by  the  action  of  suitable  reducing  agents  upon  acid 
solutions  of  platinum  salts.  When,  as  in  the  platinum 
print,  it  is  deposited  upon  the  surface  of  a  very  pure 
and  durable  paper,  there  may  be  produced  not  only  an 
effect  of  extreme  beauty,  but  probably  the  most  per- 
manent form  in  which  a  photograph  can  be  made. 
Practically  the  only  alteration  which  affects  a  properly 
made,  pure  paper  upon  which  a  platinum  image  has 
been  superposed  is  a  slight  yellowing  with  age,  due  to 


PRINTING  WITH  IRON  135 

iron  salt  which  was  imperfectly  removed  in  the  fixing 
process.  The  other  constituent  of  the  platinum  print, 
namely,  the  elementary  platinum,  is  one  of  the  most 
resistant  substances  with  which  the  chemist  has  to 
deal.  In  any  of  its  forms,  the  element  is  only  slightly 
affected  by  even  high  concentrations  of  the  single  acids, 
requiring  aqua  regia,  a  mixture  of  hydrochloric  and 
nitric  acids  (generally  in  the  proportions  of  three  to 
one)  to  dissolve  it.  It  is  fusible  only  by  use  of  the  elec- 
tric current  or  by  the  oxyhydrogen  blowpipe,  melting 
at  over  1,700  degrees  Centigrade,  is  at  no  temperature 
attacked  by  either  water  or  oxygen,  and,  except  in 
the  presence  of  alkali,  is  not  acted  upon  by  sulphur. 
Under  the  conditions  in  which  photographs  are  usually 
kept,  platinum  prints  may  therefore  be  considered  to 
be  perfectly  permanent. 

Not  including  the  preparation  of  the  coated  paper, 
platinum  printing  consists  of  the  same  three  distinct 
operations  which  have  been  described  for  the  making 
of  silver  prints,  viz.,  the  exposure  to  light,  development 
of  the  image,  and  the  fixing  of  this  image  by  removal 
of  soluble  materials.  But,  as  has  just  been  intimated, 
platinotype  is  somewhat  different  from  the  processes 
of  the  silver  print,  in  that  the  platinum  is  not  primarily 
the  image-forming  material,  but  is,  by  a  supplementary 
reaction,  substituted  for  the  original  image,  which  is,  in 
fact,  formed  in  photo-chemical ly  reduced  iron  salt. 
When  the  thin  film  of  mixed  ferric  and  platinous  salts 
spread  upon  paper  is  subjected  to  the  action  of  light, 
the  reduction  to  ferrous  salt  produces  a  very  pale  and 
unpromising  appearance,  the  picture  showing  in  a  faint 
brown  upon  a  yellow  ground.  Reaction: 

Ferric  oxalate  -f-  Light  «=  Ferrous  oxalate  -f-  Carbon  dioxide 


136    CHEMISTRY  FOR  PHOTOGRAPHERS 

The  platinum  salt,  during  and  after  the  exposure, 
itself  practically  unaffected,  is  in  the  presence  of  a  fer- 
rous compound,  but  the  two  are  inactive  towards  each 
other  in  the  dry  state.  Thus  at  this  stage  no  elementary 
platinum  is  in  the  image,  unless  by  the  access  of  moist- 
ure from  the  air  the  substances  come  into  chemical  con- 
tact by  commencing  to  dissolve.  In  order  that  the  re- 
ducing power  of  ferrous  iron  may  have  full  play  upon  the 
platinum  salt,  it  is  necessary  to  bring  the  iron  into  solu- 
tion together  with  the  platinum.  This,  with  the  water- 
insoluble  ferrous  oxalate,  which  is  the  form  of  ferrous 
salt  used,  can  be  done  by  treating  it  with  a  solution, 
which  is  used  warm  (not  below  15.5  degrees  Centigrade 
or  60  degrees  Fahrenheit),  of  potassium  oxalate,  26 
grams  of  the  salt  in  each  100  cubic  centimeters  of 
solution.  By  this  means  the  insoluble  ferrous  oxalate 
image  is  altered  to  the  soluble  double  ferrous  potassium 
oxalate.  This  at  once  reacts  with  the  dissolved  potas- 
sium chloroplatinite,  and  platinum  black  is  deposited 
by  reduction  in  place  of  the  iron  salt.  As  the  quantity 
of  platinum  reduced  is  equivalent  to  the  amount  of 
ferrous  salt,  and  this  last  is  proportional  to  the  exposure, 
the  varying  densities  of  the  silver  deposit  of  the  negative 
are  inversely  repeated  in  varying  densities  of  platinum 
black  upon  the  paper.  Reactions : 

1 .  Ferrous  oxalate  +  Potassium  oxalate 

=  Ferrous-potassium  oxalate 

2.  Ferrous-potassium  oxalate 

+  Potassium  chloroplatinite 
=  Platinum  -f-  Potassium  chloride 
+  Ferric-potassium  oxalate 

In  this  condition  the  print  is  saturated  with  a  solution 
containing  whatever  unreduced  potassium  chloroplat- 
inite is  left,  ferric-potassium  oxalate,  and  potassium 


PRINTING  WITH  IRON  137 

chloride.  All  of  these,  but  the  iron  salt  especially,  must 
be  washed  out  of  the  paper  in  order  that  no  changes 
may  later  take  place  in  the  picture.  If  the  print  were 
simply  washed  with  plain  water,  as  the  concentration 
of  the  iron  compound  diminished  the  tendency  of 
ferric  salts  to  hydrolyze  in  dilute  solutions  would  be- 
come apparent  in  the  formation  of  an  insoluble  basic 
(hydroxy-)  ferric  compound.  Reaction : 

Ferric  oxalate  +  Water 

=  Ferric  hydroxy-oxalate  +  Oxalic  acid 

No  amount  of  water  would  serve  to  effect  its  complete 
elimination  from  the  paper.  For  these  reasons,  the 
first  wash  waters  into  which  the  developed  platinum 
prints  are  put  must  contain  small  quantities  of  hydro- 
chloric acid,  about  10  cubic  centimeters  of  the  con- 
centrated acid  to  each  600  cubic  centimeters  of  water. 
Since  the  basic  ferric  compounds  are  soluble  in  hydro- 
chloric acid,  presence  of  the  acid  prevents  their  forma- 
tion. Reaction : 

Ferric  hydroxy-oxalate  +  Hydrochloric  acid 

=  Ferric  chloride  +  Ferric  oxalate 

Several  of  these  acid  iron-clearing  baths  must  be  used 
and  the  paper  passed  along  from  one  to  the  next, 
draining  each  print  as  thoroughly  as  possible  from  one 
solution  before  putting  it  into  the  succeeding  bath. 
At  least  four  or  five  of  these  clearing-solutions  should 
be  used,  prints  being  allowed  to  stay  in  the  first  one  for 
ten  minutes  and  in  each  of  the  others  for  five.  The 
last  one,  while  in  use,  must  show  no  trace  of  yellow 
color,  indicating  that  there  is  in  it  only  an  inappreciable 
amount  of  iron  salt.  After  this,  a  fifteen  minutes'  wash- 
ing in  water  will  be  ample,  or  half  an  hour  if  the  paper 
is  very  thick.  The  amount  of  ferric  salt  left  in  the 


i38    CHEMISTRY  FOR  PHOTOGRAPHERS 

print  as  it  comes  out  of  the  last  acid  clearing-bath  may 
be  indicated  by  catching  the  drainings  from  the  paper  in 
a  test  tube  and  adding  a  drop  of  ferrocyanide  solution. 

Most  of  the  soluble  platinum  salt  will  be  retained 
in  the  first  washing  tray,  and,  if  much  work  in  platinum 
is  done,  it  will  be  worth  while  to  save  not  only  this  first 
clearing-bath,  but  also  all  old  developing  solutions  and 
scraps  of  paper.  The  paper  is  burned  and  the  ashes 
saved.  Solutions  are  best  treated  by  pouring  them  all 
together  in  a  jar,  adding  a  little  hydrochloric  acid,  if 
the  mixture  does  not  already  contain  an  excess,  and 
suspending  in  it,  a  little  above  the  bottom,  some 
metallic  zinc.  The  zinc  goes  into  solution,  while  the 
platinum  separates  out.  Reactions,  using  platinic 
chloride  as  a  type : 

1 .  Platinic  chloride  +  Zinc 

=  Platinum  +  Zinc  chloride 

2.  Zinc  +  Hydrochloric  acid 

=  Hydrogen  +  Zinc  chloride 

3.  Platinic  chloride  +  Hydrogen 

=  Platinum  +  Hydrochloric  acid 

After  sufficient  standing,  the  solution  will  have  become 
decolorized,  owing  to  the  deposition  of  all  the  platinum 
and  the  reduction  to  the  ferrous  condition  of  all  the  iron 
salt.  When  this  is  the  case,  most  of  the  liquid  can  be 
carefully  decanted,  or  siphoned,  from  the  precipitate 
which  has  settled  to  the  bottom,  after  which  more  waste 
liquids  may  be  introduced  into  the  jar  and  the  recovery 
process  continued.  When  enough  solid  material  has 
accumulated,  it  is  separated  on  a  filter,  washed  with 
hot  water,  and,  combined  with  all  the  solid  residues  on 
hand  containing  platinum,  boiled  in  a  porcelain  dish 
with  aqua  regia  until  complete  solution  of  the  platinum 
is  effected.  Evaporate  the  liquid  down  on  a  water- 


PRINTING  WITH  IRON  139 

bath  adding  a  little  hydrochloric  acid  before  it  comes  to 
dryness,  and  evaporate  again,  to  boil  out  the  nitric 
acid.  Repeat  file  evaporation  to  the  crystallizing  point 
with  hydrochloric  acid  to  make  sure  that  all  nitric  acid 
is  expelled;  dilute  up  with  enough  water  and  filter  the 
solution.  Add  an  excess  of  sodium  carbonate  and  a 
little  alcohol  and  boil  the  mixture,  which  will  precipitate 
the  platinum  as  a  black  powder.  Wash  it  with  hot 
water  and  afterwards  with  hot  hydrochloric  acid  and 
dry  it  thoroughly  and  weigh  it.  Dissolve  in  aqua 
regia,  evaporate  down  with  hydrochloric  acid  several 
times  as  before  to  free  it  from  nitric  acid,  add  one  gram 
of  potassium  chloride  for  each  gram  of  platinum  present, 
and  dilute  with  just  enough  distilled  water  to  get  the 
potassium  chloroplatinate  all  in  solution.  Each  gram 
of  platinum  will  make  about  1.5  grams  of  potassium 
chloroplatinate.  The  solution  is  next  to  be  so  ad- 
justed that  for  each  two  grams  of  the  chloroplatinate 
there  will  be  25  cubic  centimeters  of  water  (distilled) 
and  one  gram  each  of  acid  potassium  sulphite  and 
potassium  chloride.  This  mixture  is  boiled  for  twenty- 
five  minutes,  counting  the  time  from  the  actual  com- 
mencement of  ebullition,  is  then  cooled,  put  in  a  shal- 
low dish,  such  as  a  thoroughly  cleaned  tray,  and  left 
to  crystallize.  Red  crystals  of  potassium  chloroplat- 
inite  will  separate  out;  they  should  be  recrystallized 
several  times  to  purify  them,  and  when  carefully  dried 
the  salt  is  ready  for  use  in  making  up  homemade 
platinotype  papers. 

The  following  formula,  suitable  for  the  so-called  "cold 
bath  development,"  is  recommended  by  Abney,  not 
only  on  account  of  the  keeping  qualities  of  the  paper 
prepared  with  it,  but  also  because,  by  variations  in  the 


i4o    CHEMISTRY  FOR  PHOTOGRAPHERS 

developer,  the  prints  are  susceptible  of  considerable 
modification : 

NO.   I 

Ferric  ammonium  oxalate 50  grams 

Water,  distilled 50  cubic  centimeters 

Oxalic  acid,  10  per  cent,  solution  15  cubic  centimeters 

NO.  z 

Potassium  chloroplatinite i  part 

Water,  distilled 5  parts 

NO.  3 

Ammonium  dichromate 4  grams 

Water,  distilled,  to 100  cubic  centimeters 

For  each  sheet  of  paper  20  inches  by  26  inches,  take  8 
cubic  centimeters  of  No.  2,  mix  with  4  cubic  centimeters 
of  No.  i,  and  8  cubic  centimeters  of  No.  3.  Unsized 
papers  must  be  given  a  good  sizing  with  gelatine  (or 
arrowroot  may  be  used) : 

Gelatine i  gram 

Water 100  cubic  centimeters 

Potassium  alum 0.3  gram 

Methyl  alcohol 25  cubic  centimeters 

Swell  the  gelatine  thoroughly  in  the  water  and  after- 
wards dissolve  it  by  heating,  then  add  the  powdered 
alum  and  methyl  alcohol.  Filter  while  hot  through  mus- 
lin, taking  care  to  avoid  bubbles.  Paper  may  be  treated 
with  the  sizing  by  immersion  or  by  coating  with  a  brush, 
in  the  manner  previously  described  in  the  present 
chapter.  In  applying  the  sensitive  coating  to  the  paper, 
it  must  be  remembered  that  we  are  dealing  with  a  very 
unstable  mixture.  For  this  reason  an  ordinary  bristle 
brush  will  not  serve,  as  it  will  soon  become  saturated 
with  the  decomposition  products  of  the  solution,  and 
these  will  be  transferred  to  other  sheets,  with  very 
disappointing  results  when  made  into  prints.  A  form 
of  brush,  figured  and  described  by  Abney,  is  made  out 


PRINTING  PROCESSES  141 

of  a  3>^  inch  by  4^  inch  piece  of  photographic  film, 
(i.e.  an  old  negative),  strips  of  cotton  flannel  to  fit  it, 
and  a  film  clip  such  as  is  used  for  tray  development  of 
roll  films.  A  strip  of  the  flannel  is  laid,  fleece  out,  upon 
the  celluloid,  and  the  ends  of  both  are  then  doubled 
back,  without  creasing  the  film,  with  the  cloth  on  the 
outside,  until  they  can  be  firmly  held  together  with  the 
clip.  The  clip  becomes  a  convenient  handle,  and  the 
resiliency  of  the  celluloid  gives  plenty  of  stiffness  without 
the  rigidity  which  would  abrade  the  paper.  For  each 
large  sheet  a  fresh  cloth  covering  must  be  used,  but 
when  smaller  sheets  of  paper  are  coated,  several  may  be 
done  before  renewing  the  flannel.  In  any  event  it  is  not 
advisable  to  use  the  same  piece  longer  than  10  to  12 
minutes.  The  sheet  of  paper  to  be  coated  may  be 
pinned,  the  sized  side  up,  at  one  end  upon  a  board  and 
grasped  at  the  other  by  tne  fingers  of  the  left  hand. 
This  will  allow  for  the  expansion  which  results  from 
wetting  it  with  the  solution.  A  measured  amount  of 
the  sensitizing  liquid,  proportioned  to  the  dimensions 
of  the  sheet  as  given  above,  is  poured  upon  the  middle  of 
the  paper  and  as  rapidly  as  possible  brushed  with  the 
spreader  in  circular  sweeps  out  into  the  corners  and 
over  the  entire  surface.  After  this  the  strokes  are  criss- 
crossed and  all  uneven  places  blended  out  as  much  as 
may  be.  The  next  part  of  the  operation  consists  in  the 
surface-drying  of  the  coated  sheet,  which  is  allowed  to 
occur  spontaneously,  but  should  not  be  achieved  in 
under  five  nor  over  ten  minutes.  If  dried  too  quickly 
the  sensitizer  will  be  liable  to  come  off  in  the  developer, 
but  if  drying  is  too  slow  it  may  soak  into  the  body  of 
the  paper,  which  produces  flat  and  foggy  pictures. 
When  surface-dry,  the  paper  is  completely  dessicated 


i4i    CHEMISTRY  FOR  PHOTOGRAPHERS 

over  a  stove  at  a  temperature  not  too  great  for  the  hand 
to  bear  (not  to  exceed  40  degrees  Centigrade),  this 
change  being  indicated  by  a  deepening  of  the  color. 
The  paper,  when  dry,  must  be  kept  dry,  which  means 
very  dry  indeed,  since  if  any  moisture  is  absorbed 
traces  of  the  iron  and  platinum  salts  immediately  go 
into  solution.  In  this  condition  not  only  is  the  ferric 
oxalate  more  unstable  and  therefore  liable  to  spon- 
taneous decomposition,  but  also  as  soon  as  the  sheet  is 
exposed  to  light  and  the  ferric  salt  begins  to  be  reduced, 
the  solubility  of  the  resulting  ferrous  salt,  although 
very  small,  enables  the  reaction  between  it  and  the 
chloroplatinite  to  proceed  at  once  and  to  a  more  or  less 
indefinite  extent,  depending  upon  the  actual  quantity  of 
water  present.  For  these  reasons  the  prints  made 
from  improperly  preserved  papers  are  liable  to  be 
granular  and  flat.  Platinum  papers  are  therefore 
commonly  stored  in  tin  cylinders  with  caps  which  can 
be  made  air-tight  by  wrapping  adhesive  tape  or  a  wide 
rubber  band  around  the  joint.  In  the  cylinder  there 
is  also  put  a  small  package  of  anhydrous  calcium 
chloride,  so  packed  that  none  of  the  lime  can  sift  out 
upon  the  paper  in  the  cylinder.  Calcium  chloride,  as  it 
crystallizes  from  water  solution,  contains  six  molecules 
of  "water  of  crystallization."  When  the  crystallized 
salt  is  heated,  at  200  degrees  Centigrade  it  loses  two- 
thirds  of  this  water  of  crystallization,  and  upon  further 
heating,  until  the  dehydrated  salt  is  melted,  the  re- 
maining third  is  driven  off,  and  the  product  is  "an- 
hydrous" calcium  chloride.  In  this  condition  it  has  a 
very  great  affinity  for  water,  so  great  that,  when  a 
little  of  it  is  exposed  upon  a  saucer  to  the  air,  in  a  very 
short  time  by  the  absorption  of  moisture  the  salt  be- 


PRINTING  WITH  IRON  143 

comes  sticky  and  soon  dissolves  in  the  accumulated 
water.     These  reactions  may  be  written: 

1 .  Calcium  chloride  crystals 

=  Calcium  chloride  (anhydrous)  +  Water 

2.  Calcium  chloride  (anhydrous)  +  Water 

=  Calcium  chloride  crystals 

It  is  by  the  application  of  the  second  reaction  that 
platinum  papers  are  preserved,  since  the  lime  salt  is  so 
much  more  hygroscopic  than  the  iron  and  platinum 
salts  of  the  paper  that  all  moisture  is  seized  and  held 
by  the  calcium  chloride.  But  there  is  naturally  a  limit 
to  the  quantity  of  water  which  can  be  absorbed  by  a 
given  amount  of  calcium  chloride.  When  this  limit 
is  approached,  its  efficiency  as  a  dehydrating  agent  is 
much  lessened,  and  of  course  when  the  limit  is  reached 
the  effectiveness  is  gone.  Therefore  in  storing  platino- 
type  paper  it  is  necessary  to  consider  the  condition  of 
the  calcium  chloride.  If  it  has  already  absorbed  much 
water,  it  can  quickly  be  dehydrated  by  heating  in  an 
iron  dish.  Otherwise  fresh  anhydrous  salt  must  be 
taken. 

For  producing  sepia  prints  in  platinotype,  mercuric 
chloride  is  included  in  the  sensitizing  solution.  The 
brown  color  of  the  image  is  due  in  some  way  to  the 
mixture  of  platinum  and  mercury  reduced  by  ferrous 
salt.  There  are  both  "cold  bath"  and  "hot  bath" 
sepia  papers,  the  latter  being  developed  in  potassium 
oxalate  solution  at  a  temperature  between  70  degrees 
Centigrade  and  80  degrees  Centigrade  (160  degrees 
Fahrenheit  to  180  degrees  Fahrenheit).  "Hot  bath" 
sepias  appear  to  be  rather  more  permanent  than  the 
others,  but  both  are  naturally  less  so  than  the  black 
papers  which  contain  nothing  else  than  pure  platinum. 


144    CHEMISTRY  FOR  PHOTOGRAPHERS 

A  sensitizer  for  "hot  bath"  sepia  paper  is  prepared 
thus: 

NO.   I 

Ferric  oxalate 40  grams 

Oxalic  acid 24  grams 

Water,  distilled,  to 200  cubic  centimeters 

NO.  2 

Potassium  chlorate 0.4  gram 

No.  i  solution,  to 100  cubic  centimeters 

NO.  3 

Potassium  chloroplatinite 10  grams 

Water,  distilled,  to 60  cubic  centimeters 

NO.  4 

Mercuric  chloride,  saturated  solu- 
tion in  distilled  water 50  cubic  centimeters 

To  coat  a  sheet  of  20  inch  by  26  inch  paper,  take  5 
cubic  centimeters  of  No.  i ,  4.7  cubic  centimeters  of  No. 
2,  9.5  of  No.  3,  and  i  .8  of  No.  4.  The  paper,  after  print- 
ing, is  developed  in  potassium  oxalate  solution  of  the 
same  concentration  as  that  used  for  the  ordinary  black 
process,  26  grams  per  i  oo  cubic  centimeters,  the  temper- 
ature being  maintained  at  70  degrees  Centigrade  to  80 
degrees  Centigrade,  as  above  noted,  over  a  gas  stove, 
or  better  on  an  electric  hot-plate.  The  acid  clearing- 
baths  are  used  at  half  the  concentration  given  for  black 
prints,  that  is,  5  cubic  centimeters  of  concentrated 
hydrochloric  acid  in  each  600  cubic  centimeters  of  the 
solution. 

The  many  modifications  to  which  platinotype  is 
susceptible  not  only  render  the  process  one  of  much 
flexibility  when  used  as  a  straight  photographic  method, 
but  also  adapt  it  especially  to  the  use  of  the  artist 
photographer.  Besides  the  control  which  is  given  by 
variations  in  the  constituents  of  the  sensitizing  solution, 
there  are  at  least  three  forms  of  control  which  can  be 
exercised  over  the  print.  In  ordinary  development  the 


PRINTING  WITH  IRON  145 

image  appears  instantly  upon  application  of  the  oxalate 
solution.  If  the  surface  of  the  exposed  sheet  is  first 
smeared  all  over  with  glycerine  and  the  print  is  then 
brush-developed,  as  the  worker  may  choose,  with  a 
mixture  of  equal  parts  of  glycerine  and  developer  or 
with  the  full-strength  developing  solution,  the  normal 
gradation  of  light  and  shade  may  be  altered  to  any 
extent  desired.  The  effect  of  the  glycerine  is  a  purely 
physical  one,  dependent  upon  the  property  of  viscosity, 
in  retarding  the  rate  of  development.  Another  me- 
chanical process,  for  treating  the  completed  print  if  it 
has  been  over-developed,  consists  in  abrading  the 
image,  or  such  portions  of  the  image  as  may  require 
local  reduction,  with  sawdust  and  water.  A  non- 
resinous  sawdust  is  needed,  and  that  from  box-wood  is 
the  best.  It  is  shaken  up  in  water  and  the  mixture  is 
poured  in  a  thin  stream  upon  the  gently  inclined  print, 
the  latter  being  supported  by  pinning  to  a  board.  In 
this  way  the  shadows  may  be  lightened  up  and  the 
darkened  highlights  cleared.  The  resultant  effect  has 
the  quality  of  an  engraving.  The  third  method  of 
control  is  chemical  and  is  the  same  in  principle  as  the 
toning  processes  which  were  discussed  under  silver 
printing.  Although,  as  we  have  said,  the  platinum 
image  is  essentially  a  very  permanent  thing  owing  to 
the  refractoriness  of  the  element,  yet  it  can  be  affected 
chemically  by  suitable  reagents,  and  for  the  purpose 
in  hand  the  same  oxidizing  agent  can  be  used  as  is 
effective  in  toning  the  silver  image,  viz.,  potassium 
ferricyanide.  Just  as  in  the  case  of  silver,  the  platinum 
is  oxidized  and  the  ferricyanide  is  reduced  to  ferro- 
cyanide.  The  toning  solution  contains  also  uranium 
nitrate  and  consequently  insoluble  red  uranium  ferro- 


1 46  CHEMISTRY  FOR  PHOTOGRAPHERS 

cyanide  is  precipitated  upon  the  image.  Since  this  is  an 
intensifying  operation,  the  print  to  be  toned  should  in 
the  first  instance  be  printed  light,  and  over-development 
is  to  be  avoided.  Brilliant  red  tones  are  secured  by  tak- 
ing special  care  to  free  the  paper  from  iron  salts  and  to 
this  end  an  extra  clearing-bath  or  two  is  advisable : 

NO.   I 

Uranium  nitrate 0.5  gram 

Acetic  acid  (glacial) 3.0  cubic  centimeters 

Water,  to 125  cubic  centimeters 

NO.  2 

Potassium  ferricyanide 0.5  gram 

Acetic  acid  (glacial) 3.0  cubic  centimeters 

Water,  to 125  cubic  centimeters 

Solution  No.  i  may  be  made  up  in  quantity  as  it  keeps 
well,  but  No.  2  is  unstable  and  must  be  prepared  just 
before  using.  Mix  the  quantity  of  No.  2  given  with  an 
equal  volume  of  No.  i  and  dissolve  in  the  mixture  o.  i 
gram  of  sodium  sulphite.  Leave  the  print  in  this 
solution  until  the  tone  is  satisfactory,  take  it  out 
quickly  and  sponge  it,  both  front  and  back,  with 
moistened  absorbent  cotton,  and  transfer  it  to  an  acid 
clearing-bath  containing  5  cubic  centimeters  of  con- 
centrated hydrochloric  acid  in  600  of  water.  Pass  the 
print  on  to  a  second  acid  bath,  and  wash  for  ten  minutes, 
and,  lastly,  lay  it  upon  clean  muslin  to  dry.  Blue  tones 
may  be  had,  from  the  same  toning  bath,  merely  by 
purposely  failing  to  clear  all  the  iron  salt  from  the 
print  immediately  after  development.  Printing  should 
in  the  first  place  be  carried  a  little  further  than  would 
give  a  normal  black  and  white  print.  After  develop- 
ment, the  print  is  put  for  but  ten  seconds  into  the 
normal  clearing  solution  and  is  then  placed  immediately 
in  the  toning  bath  until  colored  as  desired.  Clearing 


PRINTING  WITH  IRON  147 

takes  place  in  a  solution  of  half  the  concentration  used 
for  red  tones,  i.e.,  2.5  cubic  centimeters  of  hydro- 
chloric acid  in  600  of  water.  After  clearing  and  washing 
the  blue  print  is  dried  upon  muslin,  but  on  no  account 
should  the  same  pieces  of  cloth  be  used  for  both  red 
and  blue  prints,  as  the  quality  of  the  red  will  be  de- 
graded. It  is  plain  that  the  blue  color  is  due  to  ferric 
ferrocyanide  produced  by  metathesis  between  the  ferric 
salt  which  is  on  the  paper  and  the  potassium  ferro- 
cyanide reduced  from  ferricyanide  by  the  platinum. 
Success  in  the  use  of  these  toning  methods  depends 
wholly  upon  scrupulous  cleanliness  and  the  keeping  of 
blue-toned  paper  away  from  the  red. 

A  process  practically  identical  in  principle  with  plati- 
notype  is  known  as  kallitype.  Instead  of  platinum,  the 
image  is  silver,  produced  by  secondary  reduction  with 
ferrous  oxalate,  the  primary  image  being  formed  in  the 
iron  salt,  as  is  the  case  in  platinum  papers.  It  has 
been  much  complained  of  by  photographers  and  in  the 
photographic  journals  on  account  of  its  alleged  lack 
of  permanency.  There  is  no  possibility  of  doubt  but 
that  a  silver  print  by  the  kallitype  process  may  be 
made  equally  permanent  with  silver  prints  by  any  other 
method.  When  properly  worked  the  process  leaves  an 
image  in  pure  silver  upon  a  substratum  of  pure  paper, 
which  is  the  condition  essential  to  durability.  Further- 
more, the  testimony  of  time  may  be  advanced,  for  a 
certain  kallitype  print  has  hung  upon  the  wall  of  an 
ordinarily  illuminated  room  for  upwards  of  seven  years 
without  appreciable  alteration.  Any  worker  who  does 
not  get  both  permanent  results  and  exceptionally 
pleasing  pictures  with  kallitype  ought  to  test  his 
chemicals  thoroughly  and,  especially,  examine  carefully 


148    CHEMISTRY  FOR  PHOTOGRAPHERS 

his  methods  of  working,  for  somewhere  in  these  he  is 
sure  to  discover  the  cause.  There  is  one  characteristic 
of  this  printing  medium  which  ought  to  make  it  interest- 
ing to  a  great  many  amateurs,  its  exceeding  inexpensive- 
ness.  It  is  a  fact  that,  including  the  cost  of  the  chem- 
icals, paper,  developer  and  fixing  salt,  the  completed 
prints  need  not  cost  more  than  ten  cents  per  dozen  for 
the  5  inch  by  7  inch  size.  But  the  quality  which  it  is 
possible  to  obtain  in  kallitype  is  not  at  all  measured  by 
this  cheapness,  for  successful  pictures  can  scarcely  be 
told  from  platinum  prints.  Another  advantage  lies  in  the 
fact  that  negatives  which  no  other  silver  process  will  print 
satisfactorily  will  yield  excellent  pictures  in  kallitype. 

Any  kind  of  paper  may  be  used,  with  the  same 
proviso  as  in  platinotype,  that  the  unsized  varieties 
must  be  given  a  coat  of  sizing  beforehand.  The  high 
grade  drawing  papers  are  particularly  desirable;  but 
for  the  beginner  there  is  nothing  more  satisfactory  than 
a  good,  heavy  ledger  paper,  such  as  Weston's  ledger, 
since  it  is  already  well  sized,  is  very  durable,  and  has 
surface  which  gives  good  quality  in  prints.  As  the 
operations  of  sizing  the  paper  and  coating  it  with  the 
sensitizing  solution  are  the  same  as  in  platinotype,  and 
have  just  been  described,  we  may  pass  at  once  to  the 
preparation  of  the  necessary  solutions: 

NO.   I 

Ferric  oxalate 30  grams 

Water,  distilled,  to 1 50  cubic  centimeters 

Gum  arabic,  selected 3  grams 

NO.  2 

Ferric  potassium  oxalate 15  grams 

Water,  distilled,  to 240  cubic  centimeters 

NO.  3 

Oxalic  acid 15  grams 

Water,  distilled,  to 1 20  cubic  centimeters 

Ammonium  hydroxide,  concentrated     6  cubic  centimeters 


PRINTING  WITH  IRON  i4g 

NO.  4 

Potassium  dichromate 7.5  grams 

Water,  distilled,  to 1 20  cubic  centimeters 

All  these  solutions  are  to  be  made  up  and  kept  in 
chemically  clean,  dark  brown  bottles  with  tight-fitting 
glass  stoppers.  Besides  this  precaution,  they  should 
be  put  away  in  the  dark.  Number  one  is  prepared  by 
putting  the  measured  quantity  of  ferric  oxalate  in  the 
bottle,  adding  enough  water,  and  shaking  from  time 
to  time  for  half  an  hour.  After  it  has  stood  in  the  dark 
for  a  day  it  will  generally  be  found  completely  dissolved, 
and  when  this  is  the  case  the  gum  arabic  is  added  and 
left  to  dissolve.  The  solution  is  then  diluted  up  to  the 
specified  volume  with  water.  It  will  keep  for  several 
months.  The  other  three  solutions  need  no  comments. 
The  sensitizing  mixture  for  normal  negatives  consists  of: 

No.  i  solution 30  cubic  centimeters 

No.  2  solution 15  cubic  centimeters 

No.  3  solution 1.8  cubic  centimeters 

No.  4  solution 4  drops 

Silver  nitrate,  pure 2.2  grams 

About  2  cubic  centimeters  of  the  mixture  are  required 
to  coat  a  sheet  1 1>£  inches  by  18  inches.  Free  nitric 
acid  in  the  silver  nitrate  is  detrimental  and  should  be 
tested  for  with  litmus  paper  beforehand.  The  salt  may 
be  purified  by  recrystallization,  if  necessary.  Special 
sensitizers  can  be  prepared  for  different  kinds  of  nega- 
tives. For  example,  for  very  contrasty  negatives,  No.  4 
may  be  omitted  and  No.  2  and  No.  3  increased,  and 
more  silver  nitrate  added.  Too  great  increase  in  the 
proportion  of  No.  3  induces  fog.  For  negatives  that 
are  very  soft  and  thin  No.  4  may  be  increased,  diminish- 
ing the  quantity  of  No.  3,  However,  it  is  generally 
better  to  use  a  uniform  sensitizer  and  exercise  the 


150    CHEMISTRY  FOR  PHOTOGRAPHERS 

necessary  control  by  modifications  in  printing  and 
development.  The  coated  paper  will  keep  in  good 
condition  for  a  month  without  any  special  precautions 
except  against  exposure  to  light.  If  it  is  desired  to 
preserve  it  longer,  it  may  be  stored  in  the  same  way 
as  platinum  paper  with  anhydrous  calcium  chloride  as 
a  dehydrating  agent. 

After  printing,  which  is  similar  to  platinum,  develop- 
ment is  brought  about  in  a  solution  of  sodium  acetate, 
variously  modified  according  to  the  tone  which  is 
wanted.  The  acetate  solution,  125  grams  of  the  salt 
dissolved  and  diluted  to  one  liter,  can  be  made  up  in 
bulk,  as  it  works  best  when  a  day  or  two  old.  For 
warm  blacks,  take : 

Acetate  solution 240  cubic  centimeters 

Tartaric  acid 0.75  gram 

No.  4  solution 0.6  to  6.0  cubic  centimeters 

The  longer  the  exposure  or  the  flatter  the  negative,  the 
greater  should  be  the  proportion  of  No.  4  solution  used, 
as  potassium  dichromate  appears  to  act  somewhat  as  a 
restrainer.  For  platinum  black  tones  take: 

Acetate  solution 240  cubic  centimeters 

Tartaric  acid 1.5  gram 

Phosphoric  acid  (50  per  cent.)          i .  2  cubic  centimeters 
No.  4  solution 0.6  to  3.0  cubic  centimeters 

To  make  sepia  prints  with  kallitype,  the  developer 
contains  borax  and  Rochelle  salts : 

NO.  5 

Water,  distilled,  hot,  to 1000  cubic  centimeters 

Borax,  powdered 37.5  grams 

Sodium-potassium  tartrate .  .  .  75  grams 

NO.  6 

Sodium-potassium  tartrate.  . ,         75  grams 
Water,  distilled,  to 1000  cubic  centimeters 


PRINTING  WITH  IRON  151 

In  making  up  No.  5,  dissolve  the  borax  in  the  hot 
water,  let  it  cool,  and  finally  add  the  tartrate,  and  dilute 
to  one  liter.  Sepia  tones  are  obtained  by  mixing  these  so- 
lutions in  the  proportion  of  one  part  of  No.  5  to  three 
parts  of  No.  6,  and  adding  No.  4  solution  as  required. 

Kallitype  prints  must  be  both  cleared  of  iron  salts 
and  fixed  in  thiosulphate  solution,  and  it  is  in  the  last 
operation  that  the  process  differs  from  platinotype. 
The  kind  of  clearing-bath  to  use  depends  upon  the 
developer.  Following  immersion  in  sodium  acetate 
solution,  prints  are  cleared  in  a  bath,  made  fresh  each 
time: 

Sodium  citrate 7.5  grams 

Citric  acid 1.5  gram 

Water,  to 250  cubic  centimeters 

After  the  borax  and  Rochelle  salts  developer,  use  a 
solution  of  Rochelle  salts,  75  grams  to  the  liter  of  water, 
to  which  are  added  7.5  cubic  centimeters  of  No.  4  solu- 
tion. Leave  the  prints  in  this  bath  for  ten  minutes. 
After  coming  out  of  the  clearing-bath,  they  must  be 
thoroughly  washed  to  free  them  from  all  traces  of  acid, 
since  this  would  cause  decomposition  of  the  thiosul- 
phate fixing  bath  which  follows.  They  are  then  fixed  for 
ten  minutes  in : 

Sodium  thiosulphate 25  grams 

Water 500  cubic  centimeters 

Concentrated  ammonium  hydroxide      5  cubic  centimeters 

and  given  a  final  thorough  washing  until  all  thiosulphate 
has  been  removed.  After  this  the  prints  are  dried, 
best  between  lintless  photographic  blotters,  to  keep 
them  free  from  dust. 

In  this  chapter  the  attempt  has  been  made  principally 
to  give  an  outline  of  the  processes  with  ferric  salts, 
showing  as  simply  as  possible  the  rationale  of  the 


152    CHEMISTRY  FOR  PHOTOGRAPHERS 

methods  and  describing  in  an  elementary  way  the 
chemical  reactions  upon  which  they  depend  for  making 
technically  successful  prints.  It  is  possible  that  some 
readers,  at  least,  may  become  sufficiently  interested 
to  wish  to  pursue  the  subject  further.  Such  may  find 
in  the  accompanying  bibliography  full  and  practical 
accounts  for  handling  these  fascinating  and  beautiful 
photographic  media. 


CHAPTER  X 
PRINTING  PROCESSES  WITH  CHROMIUM  SALTS 


ORGANIC  colloids,  such  as  gelatine,  gum  arabic, 
etc.,  have  the  property  of  combining  with  chro- 
mium salts  (in  which  the  element  is  positive,  or  basic), 
with  mercurous  chloride,  tannic  acid,  and  various 
resins.  The  reaction  is  apparently  not  a  photo- 
chemical one,  and  its  product  is  the  colloid  so  altered 
by  the  chemical  admixture  of  chromium  salt,  to  take 
an  example,  as  no  longer  to  be  soluble  in  water.  That 
this  insoluble  compound  has  photo-chemical  properties 
is  shown  by  the  fact  that,  if  it  is  not  exposed  to  light, 
it  is  able  to  combine  with  water,  becoming  hydrated  and 
swelling,  as  the  water  is  absorbed,  in  a  similar  manner 
to  the  original,  uncombined  colloid;  whereas  after  it 
has  been  exposed,  not  only  is  it  still  insoluble,  but  it 
refuses  to  absorb  any  water,  that  is,  does  not  become 
hydrated.  When  the  colloid,  however,  is  mixed  with  a 
chromate  (usually  a  dichromate)and  kept  in  the  dark, 
no  insolubilizing  action  takes  place.  If  the  solution  is 
coated  upon  paper,  and  not  exposed  to  light,  it  will 
quickly  be  dissolved  and  washed  off  by  warm  water. 
But  by  the  action  of  light  upon  the  mixture,  reduction 
of  the  chromium,  on  the  one  hand,  and  partial  oxidation 
of  the  colloid,  on  the  other,  occur;  and  these  reaction 
products  are  chemically  combined,  forming  the  in- 
soluble, non-absorbing  end-product  of  colloid-chromium 
salt.  The  oxidizing  action  of  chromates  upon  organic 
compounds  is  well  illustrated  by  the  accompanying 
experiment. 


154    CHEMISTRY  FOR  PHOTOGRAPHERS 

Experiment  34.  Acidify  with  hydrochloric  acid  5  or 
10  cubic  centimeters  of  dilute  potassium  dichromate 
solution,  heat  the  liquid  to  boiling,  add  a  little  ethyl 
alcohol,  and  continue  the  boiling.  The  gradual  change 
in  the  color  of  the  solution  from  orange  to  green  in- 
dicates the  reduction  of  the  chromate  to  chromic 
chloride;  and  at  the  same  time  the  pungent  odor  of 
aldehyde  shows  the  oxidation  of  alcohol.  Reaction: 

Potassium  dichromate  +  Ethyl  alcohol 

+  Hydrochloric  acid 

=  Chromic  chloride  +  Potassium  chloride  +  Ethyl 

aldehyde  +  Water 

In  chromic  acid  and  its  salts,  such  as  the  dichromate 
for  example,  the  chromium  atom  forms  part  of  the 
acidic  or  negative  portion  of  the  molecule ;  but  in  such 
compounds  as  chromic  chloride,  chrome  alum,  etc., 
chromium  is  in  the  basic  or  positive  part.  Reduction  of 
a  chromate,  therefore,  as  illustrated  above,  consists  in 
changing  the  chromium  atom  from  the  negative  part 
of  the  compound  to  the  positive  part,  as  well  as  in 
removing  oxygen  from  it.  We  are  now  ready  to  ex- 
periment with  the  action  of  light-reduced  chromium 
salt  upon  gelatine. 

Experiment  35.  Soak  up  in  enough  cold  water  20 
grams  of  gelatine  and  get  it  into  solution  by  warming. 
In  a  weak  light  dissolve  in  this  6  grams  of  potassium 
dichromate  and  dilute  up  to  100  cubic  centimeters. 
Coat  some  of  this  sensitive  mixture  upon  several  small 
pieces  of  paper  and  dry  them.  Expose  three  of  these 
under  a  contrasty  negative,  giving  full  exposure,  and 
treat  them  severally  as  follows: 

(i)  Place  one  of  the  prints  in  hot  water.  The 
gelatine  dissolves  off  from  the  paper  in  proportion  as  it 


PRINTING  WITH  CHROMIUM          155 

has  been  protected  from  the  light,  but,  wherever  light 
has  acted,  more  or  less  gelatine  has  become  insoluble 
and  non-absorbent,  according  to  the  amount  of  the 
light-effect. 

(2)  Put  a  second  print  into  cold  water  and  observe 
that  the  areas  which  were  protected  from  light  by  the 
dense  parts  of  the  plate  swell,  by  the  absorption  of 
water,  the  light-affected  portions  being  unchanged. 

(3)  Spread  upon  the  third  exposed  paper  some  thin 
printers'  ink,  soak  it  for  a  time  in  cold  water,  and  then 
gently  wipe  the  surface  with  a  wet  sponge  or  wet 
absorbent  cotton.     Upon  those  areas  which  were  af- 
fected by  the  light,  since  they  have  been  rendered  non- 
absorbing,  the  ink  adheres.     From  the  other  parts  it  is 
removed  according  to  the  amount  of  water  absorption, 
which  is  proportional  to  the  photo-chemical  effect. 

In  these  simple  experiments  -we  have  the  basis  of  all 
the  processes  which  depend  upon  the  dichromatizing  of 
colloids,  and  we  must  now  try  to  make  clear  not  only 
what  happens  in  the  photo-chemical  reaction,  but  ex- 
actly what  conditions  prevail  at  the  end  of  the  reaction 
in  the  thin  layer  of  medium  spread  upon  the  surface 
of  the  paper.  When  gelatine  and  a  chromic  compound 
are  mixed  they  combine,  as  we  have  previously  noted, 
and  an  insoluble  product  results  which  is  water-ab- 
sorbing, if  unexposed,  but  non-absorbirfg  after  ex- 
posure. This  reaction  may  perhaps  be  expressed  in 
the  following  way,  in  the  absence  of  light  : 

Gelatine  +  Potassium-chromic  sulphate 

=  Gelatine-chromic  sulphate  +  Potassium  sulphate 

A  mixture  of  gelatine  and  a  salt  of  chromic  acid,  al- 
most insensitive  in  solution,  or  before  the  paper  is  dry, 
when  dehydrated  appears  to  form  a  photo-sensitive 


156    CHEMISTRY  FOR  PHOTOGRAPHERS 

compound.  It  is  certain  that  the  dried  mixture,  as  we 
learned  in  the  experiment,  is  sensitive.  The  combina- 
tion may  perhaps  be  represented  thus : 

Gelatine  +  Potassium  dichromate 

=  Gelatine-potassium  dichromate 

By  light,  this  complex  compound  is  so  altered  that 
hydrogen  is  abstracted  from  the  gelatine  portion 
(oxidation)  and  oxygen  from  the  chromium  part  (re- 
duction), and  the  final  product  is  both  insoluble  and  a 
non-absorbent  of  water : 

Gelatino-potassium  dichromate  +  Light 

=  Oxidized-gelatino-chromic  oxide  +  Potassium 

hydroxide 

At  all  events,  it  is  practically  certain  that  the  par- 
tially oxidized  gelatine  and  the  reduced  chromium  com- 
pound are  in  chemical  combination  in  the  end.  The  con- 
ditions prevailing  in  the  sensitive  gelatino-dichromate 
film  during  the  exposure  can  now  be  considered.  Let 
us  in  imagination  isolate  one  ray  of  the  white  light  and 
allow  it  to  fall  upon  different  portions  of  the  negative 
at  will.  When  it  is  directed  upon  an  area  of  clear 
glass,  the  instant  it  reaches  the  gelatine  surface,  ab- 
sorption of  its  energy  commences  and  the  photo- 
chemical change  is  initiated.  Consequently,  as  the 
layer  of  gelatine  immediately  beneath  the  surface  is 
met,  there  is  a  diminished  amount  of  light-energy 
applied  to  it,  and  so  on,  through  layer  after  layer,  until 
either  the  film  is  entirely  passed  through  or  the  light- 
•energy  is  wholly  absorbed.  Of  course,  it  must  be  under- 
stood that  it  is  the  energy  of  the  photo-chemical,  or 
actinic,  wave-lengths  of  which  we  are  speaking.  In 
the  case  of  the  yellow  dichromated  gelatine  the  energy 
of  the  actinic  wave-lengths  is  rapidly  used  up,  that  is, 


PRINTING  WITH  CHROMIUM          157 

the  light  is  able  to  penetrate  but  a  few  layers  before 
being  completely  absorbed.  In  areas  partially  shaded 
by  thin  deposits  of  silver,  much  of  the  light  has  already 
been  absorbed  by  the  negative  before  arriving  at  the 
gelatine  surface  of  the  paper,  therefore  the  depth  to 
which  its  influence  extends  in  the  film  is  less,  in  pro- 
portion to  the  amount  of  absorption  by  the  negative, 
than  in  the  unshaded  parts.  I  f  in  any  regions  the  silver 
deposit  of  the  plate  is  sufficiently  dense  to  absorb  all 
actinic  wave-lengths,  no  effect  upon  the  gelatine  is 
produced.  Dichromated  gelatine  is  sensitive  to  longer 
wave-lengths  than  are  the  silver  salts,  and  its  maximum 
occurs  in  the  blue-green.  Since  most  of  the  photo- 
chemical action  is  caused  by  the  blue  rays,  it  will  be 
clear  that  only  a  very  thin  layer  of  film  can  be  affected 
under  the  best  conditions  of  exposure  and  that  but 
little  preliminary  absorption  by  the  silver  image  makes 
marked  differences  in  the  depth  to  which  the  light- 
effect  penetrates.  Usually  the  whole  of  the  surface 
layer  of  gelatine-coated  paper  is  insolubilized  and  the 
still  soluble  portions  are  consequently  enclosed  between 
this  layer  and  the  paper  support,  where  they  are 
protected  from  the  water  in  which  the  prints  are  de- 
veloped, as  was  remarked  in  Chapter  III. 

This  brings  us  naturally  to  the  subject  of  carbon 
printing,  for  the  situation  just  described  is  the  cause  of 
the  marked  difference  between  the  manipulation  of 
carbon  and  that  of  silver  and  platinum  prints.  The 
principles  which  have  been  outlined  in  the  preceding 
chapters  are  applied,  in  the  carbon  process,  as  well  as 
in  the  other  colloid-dichromate  methods,  by  incorporat- 
ing with  the  gelatine-dichromate  mixture  a  permanent 
pigment  in  suitable  proportion.  This  coloring  matter 


158    CHEMISTRY  FOR  PHOTOGRAPHERS 

may  be  the  form  of  carbon  known  as  "lamp-black,"  or 
it  may  be  almost  any  water-color  pigment  desired. 
The  photographic  image  is  formed  by  the  finely  divided 
particles  of  this  pigment  embedded  in  insolubilized 
gelatine,  since  in  all  parts  of  the  sensitive  film  where 
the  gelatine  remains  soluble,  owing  to  its  not  having 
been  acted  upon  by  light,  the  washing  away  of  the 
gelatine  removes  also  the  pigment,  but  wherever  the 
gelatine  is  insoluble  it  firmly  retains  the  coloring 
material.  In  working  the  carbon  process  it  is  scarcely 
to  be  recommended  that  the  amateur  attempt  the 
preparation  of  "carbon  tissue,"  as  it  is  called.  A  much 
better  and  more  uniform,  and  therefore  more  satis- 
factory, product  can  be  purchased  from  the  dealers. 
These  tissues  come  provided  with  the  gelatine-pigment 
film  in  a  very  considerable  range  of  colors  and  either 
cut  to  size  or  in  rolls.  Before  printing  they  are  sen- 
sitized by  immersion  of  the  tissue  in  a  solution  of  di- 
chromate,  and  dried  in  the  dark  : 

Potassium  dichromate 1 5  to  30  grams 

Ammonium  carbonate 1.5  grams 

Water,  to i  ooo  cubic  centimeters 

The  proportion  of  dichromate  is  varied  to  suit  climatic 
conditions,  less  being  used  in  summer,  or  in  a  warm 
climate,  than  is  needed  in  winter,  or  in  a  cold  climate. 
The  function  of  ammonium  carbonate  is  to  aid  in  pre- 
serving the  gelatine  from  becoming  spontaneously  in- 
soluble, and  it  is  slightly  variable  in  amount.  Less 
may  be  used  in  cool,  dry  weather,  and  more  if  it  is 
damp.  Alcohol,  or  a  mixture  of  alcohol  and  ether,  is 
sometimes  recommended  to  be  added  to  the  sensitizing 
solution  to  secure  rapid  drying  in  hot  weather  when  the 
humidity  is  high.  Such  a  formula  is  the  following : 


PRINTING  WITH  CHROMIUM          159 

Potassium  dichromate. ...  30  grams 

Ammonium  hydroxide,  con-          0.75  cubic  centimeters 

centrated (about  1 2  drops) 

Water 700  to  800  cubic  centimeters 

To  this  add  a  mixture  of  ethyl  ether,  30  cubic  centi- 
meters, and  ethyl  alcohol,  15  cubic  centimeters,  and 
make  up  the  solution  to  1,000  cubic  centimeters  with 
water.  Keep  all  sensitizing  solutions  away  from  the 
light. 

When  sensitized,  care  has  to  be  taken  that  no  actinic 
light  reaches  the  tissue,  as  it  has  been  found  that  once 
the  reaction  is  started  by  light  it  will  proceed  slowly 
even  in  the  dark;  and  for  this  reason  when  carbon 
tissue  is  exposed  it  is  at  once  developed.  If  it  be 
under-exposed,  however,  and  put  away  in  the  dark  for 
a  suitable  length  of  time,  on  account  of  this  continuing 
action  a  normal  print  will  be  developed  up.  No  visible 
image  is  formed  in  printing,  so  that  the  exposure  is 
regulated  by  actinometer.  In  order  to  develop  the 
printed  tissue  by  washing  away  the  soluble  portions 
of  pigmented  gelatine,  since  these  soluble  layers  lie 
between  the  insoluble  upper  surface  and  the  paper 
backing,  it  is  necessary  to  remove  the  backing.  This 
operation  is  performed  by  putting  into  cold  water  the 
exposed  tissue  and  with  it  either  a  sheet  of  "single 
transfer"  paper  or  waxed  "temporary  support"  accord- 
ing as  the  picture  is  to  come  out  finally  reversed  as  to 
right  and  left,  or  unreversed.  The  transfer  paper  has 
a  gelatine  coating  upon  one  side  and  this  is  placed  face 
to  face  with  the  tissue.  Both  are  drawn  together  from 
the  water  and  are  firmly  squeegeed  into  contact,  all  air 
and  water  being  expelled  from  between  them.  The 
united  sheets  are  next  put  between  blotting  papers 
under  pressure,  and  left  from  a  quarter  of  an  hour  to  over 


160    CHEMISTRY  FOR  PHOTOGRAPHERS 

an  hour,  depending  upon  the  pressure  used  and  the 
character  of  the  paper's  surface,  in  order  that  the  tissue 
and  transfer  paper  may  adhere  together  uniformly. 
As  soon  as  the  dichromate  in  the  tissue  penetrates 
to  the  back  of  the  transfer  paper,  staining  it  yellow, 
development  may  be  undertaken.  This  part  of  the 
process  consists  merely  in  a  preliminary  soaking  of  the 
combined  papers  in  water  at  a  temperature  between 
32  degrees  Centigrade  and  38  degrees  Centigrade.  The 
gelatine  around  the  edges  presently  begins  to  dissolve. 
After  a  few  minutes  longer,  put  the  papers  into  a  second 
tray  of  water,  arranged  to  be  kept  at  a  temperature 
from  38  degrees  Centigrade  to  48  degrees  Centigrade, 
and  in  a  minute  or  two,  beginning  at  one  corner,  strip 
off  the  tissue.  The  print,  which  is  now  to  be  left  upon 
the  transfer  paper,  must  be  held  under  the  surface  of 
the  water  during  this  process  of  stripping.  By  rocking 
the  tray,  the  solvent  action  of  the  warm  water  is  caused 
to  begin  removing  the  soluble  gelatine,  and  details 
commence  to  appear  in  the  picture.  Development  is 
completed  from  this  point  by  laying  the  print  face  up  on 
a  plate  of  glass  inclined  over  the  tray  and  gently  pouring 
the  warm  water  over  it  with  a  cup.  The  print  dries 
slightly  darker  than  it  looks  when  wet,  so  it  is  developed 
to  a  little  lighter  tint  than  is  desired  in  the  final 
picture,  and  is  placed  in  cold  water  as  a  stop-bath. 
After  this  it  is  allowed  to  soak  in  a  five  percent,  solu- 
tion of  potassium  aluminum  sulphate  ("alum")  for  ten 
minutes  up  to  an  hour,  according  to  the  thinness  or 
thickness  of  the  paper,  and  is  lastly  washed  in  running 
water  for  half  an  hour.  If  right-and-left  reversal  is 
not  objectionable,  as  in  pictorial  work  and  some 
portraiture,  and  the  "single  transfer"  paper  has  been 


PRINTING  WITH  CHROMIUM          161 

used,  the  picture  is  finished  by  drying.  Otherwise  the 
"double  transfer"  method  will  have  been  employed, 
and  the  thoroughly  washed  print  has  finally  to  be 
transferred  again,  from  the  "temporary  support,"  to 
the  final  support  on  "double  transfer"  paper.  This 
paper,  also  gelatine-coated,  is  given  a  preliminary  soak- 
ing in  water  at  3 2*  degrees  Centigrade  to  38  degrees 
Centigrade  to  soften  up  the  gelatine,  -and  is  afterwards 
put,  together  with  the  print,  into  cold  water.  Face 
together,  the  two  are  withdrawn  from  the  tray,  avoiding 
air-bubbles,  then  they  are  squeegeed  into  contact  and 
hung  up  to  dry.  When  thoroughly  dried,  in  six  hours 
or  so,  the  two  papers  are  separated  at  one  corner  and 
stripped  apart.  The  gelatine  image  leaves  the  waxed 
surface  of  the  "temporary  support"  and  sticks  perma- 
nently to  the  final  support,  upon  which  it  is  dried.  In 
this  way  a  non-reversed  picture  is  obtained.  Waxing  of 
the  "temporary  support"  is  accomplished  by  spreading 
with  a  tuft  of  cotton  upon  the  gelatine  surface  a  solution 
of  20  grams  of  yellow  resin  and  7  grams  of  beeswax 
in  400  cubic  centimeters  of  turpentine.  Afterwards  it 
has  to  be  polished,  with  a  larger  wad  of  cotton,  and 
when  all  the  turpentine  has  evaporated  the  paper  is 
ready  for  use.  If  carbon  prints  are  made  from  enlarged 
negatives,  the  process  can  be  simplified  by  making  the 
enlargement  a  reversed  negative,  when  the  "single 
transfer"  method  will  give  a  non-reversed  picture. 
The  published  accounts  and  descriptions  of  carbon 
printing  all  sound,  in  the  reading,  much  more  com- 
plicated than  the  process  actually  is.  This  fact, 
together,  perhaps,  with  the  seemingly  unorthodox 
bandying  about  of  the  photographic  image,  turning  it 
upside  down  for  development  and  putting  it  back 


i6i    CHEMISTRY  FOR  PHOTOGRAPHERS 

right-side  up  again,  has  probably  deterred  a  great 
many  amateurs  from  experimenting  with  it.  This  is 
unfortunate,  for,  even  if  we  admit  the  charge  of  greater 
complication,  one  really  well-executed  print  in  carbon, 
or  in  gum,  is  worth  a  whole  handful  of  the  usual  de- 
veloping-out  prints  which  so  many  amateurs  seem  to 
be  fond  of  turning  out  wholesale.  The  point  to  be 
established  here  is  not  that  beautiful  pictures  are 
impossible  in  silver,  whether  on  printing-out  or  on 
developing-out  papers,  for  they  are  not  impossible,  but 
that  by  reason  of  the  widely  advertised  simplification 
of  photography  for  the  amateur,  the  art  has  become 
too  automatic  and  stereotyped.  All  his  thinking  and 
study  have  been  performed  for  him  by  the  manu- 
facturers. The  real  photographic  education  of  the 
amateur  generally  begins  only  when  he  takes  up  one  or 
other  of  the  individualistic  printing  methods,  platinum, 
carbon,  gum,  etc.,  and  endeavors  to  express  something 
in  his  pictures. 

Printing  in  gum,  or  gum-bichromate  as  it  has  been 
called,  is  founded  on  the  same  principles  as  carbon, 
since  it  is,  in  fact,  a  variation  of  the  carbon  process. 
A  mixture  of  gum  arabic,  potassium  dichromate,  and 
pigment  is  coated  upon  paper,  dried,  exposed  to  light, 
and  washed  in  water  and  dried.  The  result,  if  success- 
ful, has  a  distinctiveness  which  has  commended  the 
method  to  the  best  pictorialists  the  world  over.  In 
working  gum,  the  photographer  theoretically  can  ac- 
quire absolute  command  over  the  process,  since  the 
choice  of  paper  and  pigment  is  entirely  under  his 
control,  the  proportions  in  which  they  may  be  put 
together  with  the  colloid  can  be  varied  to  suit  his 
requirements,  ajid  the  development  process  may  be 


PRINTING  WITH  CHROMIUM          163 

such  as  to  allow  unlimited  scope  to  his  artistic  abilities. 
It  is  not  a  complicated  process,  at  least  in  single  print- 
ing; requires  no  transfer  of  the  colloid-pigment  image; 
is  capable  of  giving  as  fine  prints  as  any  other  method 
(some  think  finer) ;  and  withal  is  so  exceedingly  inex- 
pensive that  the  amateur  can  experiment  with  it  with- 
out stint.  If  he  is  so  unfortunate  as  to  fail  to  get  satis- 
factory pictures  in  the  end,  he  can  console  himself  with 
the  thought  that  he  is  out  little  besides  the  time  spent. 
But  there  is  no  need  for  anyone  to  fail,  for  various 
methods  have  been  published  for  the  gum  print,  and  it 
is  necessary  only  to  study  out  carefully  the  reasons  for 
the  different  manipulations  and  then  apply  patience  and 
pains  in  performing  them  for  oneself  in  order  to  make 
prints  that  at  the  very  least  will  bear  inspection. 

The  essential  materials  for  working  in  gum  are  four 
in  number,  paper,  which  may  be  of  different  weights, 
surfaces,  and  textures,  but  must  be  sufficiently  tough 
to  withstand  soaking  in  water;  gum  arabic,  suspended 
in  a  muslin  bag  in  cold  water  in  the  proportion  of  40 
grams  of  the  gum  to  60  cubic  centimeters  of  water, 
and  to  which  is  added  formaldehyde  10  drops  for  each 
i  oo  cubic  centimeters,  or  0.02  gram  of  mercuric  chloride ; 
a  saturated  solution  (at  ordinary  temperatures)  of 
potassium  dichromate  kept  in  a  brown  bottle;  and, 
for  the  beginner,  two  or  three  pigments  such  as  lamp- 
black, burnt  sienna,  and  burnt  umber,  or,  for  the 
advanced  worker,  in  addition  to  the  foregoing,  ivory 
black,  Indian  red,  Venetian  red,  Indian  yellow,  per- 
manent blue,  cobalt  blue,  madder  lake,  and  emerald 
oxide  of  chromium,  etc.  Only  permanent  colors 
should  be  selected,  and  on  this  account  the  aniline  dyes 
are  discarded.  It  is  immaterial  whether  the  pigments 


1 64  CHEMISTRY  FOR  PHOTOGRAPHERS 

are  in  water-color  tubes  or  in  powder  form,  the  ad- 
vantage of  cheapness  being  with  the  latter.  Of  this 
list  of  colors,  lamp-black,  ivory  black,  burnt  sienna, 
Indian  yellow,  cobalt  blue,  madder  lake,  and  emerald 
oxide  of  chromium  are  transparent;  burnt  umber, 
Venetian  red,  and  permanent  blue  are  semi-trans- 
parent; and  Indian  red  is  opaque.  Besides  these  four 
essentials  there  are  several  accessories  needed.  For 
coating  the  emulsion  upon  paper,  the  best  method  is 
by  use  of  the  "air-brush."  It  gives  the  most  uniform 
coating,  without  streaks  or  brush-marks,  and  by  it 
the  whole  sheet  is  coated,  without  being  cut  up.  In 
lieu  of  this  desirable  implement,  ordinary  brush-coat- 
ing will  do.  Use  a  flat  brush  that  is  soft  and  thick  and 
about  four  inches  in  width,  or  narrower  for  smaller 
sizes  of  paper,  and  even  up  streaks,  etc.,  by  blending 
with  a  three-  or  four-inch  flat  badger  blender.  The 
coating  mixture  is  made  by  grinding  the  ingredients 
together  in  a  mortar  with  a  capacity  of  about  200  cubic 
centimeters.  To  make  this  mixture,  put  about  equal 
quantities  (by  volume)  of  pigment  and  gum  solution 
into  the  mortar  and  grind  them  together  with  the  pestle. 
If  any  of  the  pigment  fails  to  be  taken  up  by  the  gum, 
add  just  enough  more  of  the  latter  to  take  up  in  sus- 
pension the  rest  of  the  former.  In  this  way  the  gum 
and  pigment  are  to  be  balanced  against  each  other, 
there  being  an  excess  of  neither.  Complete  the  mixture 
by  the  addition  of  approximately  four  to  six  times  the 
bulk  of  gum-pigment  of  the  saturated  dichromate,  mix 
thoroughly  in  the  mortar,  and  strain  through  muslin. 
Coat  and  dry  the  paper,  and,  if  it  is  to  be  kept  any 
length  of  time,  avoid  exposing  it  to  any  actinic  light 
after  it  becomes  dry.  So  long  as  it  is  wet,  it  remains 


PRINTING  WITH  CHROMIUM          165 

insensitive,  but  it  becomes  about  as  sensitive  as  plat- 
inum paper  as  soon  as  the  moisture  is  removed,  and, 
on  account  of  the  continuing  action  that  has  previously 
been  described,  even  short  exposure  to  actinic  light 
in  this  condition  produces  after  a  time  general  fog 
when  development  takes  place.  The  sensitized  paper 
will  keep  for  some  days  in  good  condition  if  unexposed 
and  stored  where  it  will  be  thoroughly  dry.  The  ex- 
posure of  paper,  coated  as  described,  is  a  simple  matter  if 
pigments  other  than  browns  and  blacks  are  used,  since 
the  paper  prints  out.  Give  a  full  printing  to  bring  out 
all  detail  and  gradation.  When  using  browns  and 
blacks,  of  course  there  will  be  no  visible  image,  and  the 
printing  time  has  to  be  determined  by  actinometer. 
Development  is  performed  by  means  of  wet  blotting- 
paper  of  the  photographic  quality.  First  soak  the 
cut-to-size  blotters  thoroughly  so  that  all  bubbles  of 
air  may  be  expelled.  Lay  down  a  clean  sheet  of  glass 
the  same  dimensions  as  blotters  and  prints,  and  upon 
it  put  one  of  the  wetted  blotters.  Soak  the  prints  for 
a  few  minutes  in  cold  water,  separately,  and  put  one 
face  down  upon  the  wet  blotter,  the  second  one  face 
up  upon  the  first,  then  another  wet  blotter  and  two 
more  prints  back  to  back,  and  so  on  until  all  the  prints 
are  in  the  pile,  with  a  blotter  upon  the  top,  and  finished 
off  with  a  second  plate  of  glass.  Cover  the  entire  pile 
with  water  in  a  sufficiently  deep  dish  and  allow  the 
developing  process  to  go  on  automatically.  The 
length  of  time  required  will  depend  upon  depth  of 
printing,  prints  that  have  been  most  fully  exposed 
developing  slowest.  Lightly  printed  paper  may  be 
completely  developed  in  an  hour  or  so.  Consequently, 
until  one  has  acquired  the  knack  of  judging  before- 


1 66  CHEMISTRY  FOR  PHOTOGRAPHERS 

hand  about  when  his  prints  will  be  ready,  it  will  be 
well  to  examine  them  after  about  an  hour.  This  task 
is  done  by  taking  the  whole  pile  of  glass,  prints,  and 
blotters  in  one  body  from  the  water  and,  resting  it  on 
one  corner,  letting  it  drain  for  several  minutes  until  the 
superfluous  water  has  run  away.  Take  down  the  pile, 
setting  it  up  again  in  the  reverse  order  as  fast  as  it 
comes  down,  but  removing  and  drying  any  prints  that 
appear  to  be  fully  developed.  They  will  dry  a  trifle 
darker  than  they  appear  when  wet.  Then  put  the 
unfinished  ones,  in  the  pile,  back  to  soak,  and  examine 
again  in  an  hour,  or  longer  if  they  were  thought  to 
need  more  time  than  that.  Prints  will  sometimes  be 
put  to  soak  without  being  examined,  needing  no  atten- 
tion in  the  interim,  over  night,  or  for  twenty-four 
hours.  The  blotting-paper  can  be  used  over  again, 
up  to  three  times,  if  it  is  not  permitted  to  dry  until 
the  absorbed  gum  has  been  washed  out,  and  it  is  then 
kept  from  the  light.  The  method  here  briefly  outlined 
is  due  to  Mr.  Walter  Zimmerman,  to  whom  photog- 
raphers both  professional  and  amateur  are  indebted 
for  many  helpful  suggestions  besides  his  excellent 
gum  process.  A  much  more  complete  and  detailed 
account  of  it  is  given  in  the  Photo-Miniature,  Number 
113,  "Gum-Bichromate  Printing,"  which  the  reader 
is  earnestly  advised  to  study. 

There  are  several  interesting  modifications  of  colloid- 
dichromate  methods,  two  of  which  are  the  inventions 
of  Mr.  Thomas  Manly.  In  the  ozotype  process  a  gela- 
tine film  impregnated  with  dichromate,  manganous 
salts,  alum,  and  boric  acid  is  prepared  upon  paper  and 
dried  in  the  dark.  The  mixture  of  alum  and  boric 
acid  acts  as  a  preservative.  By  the  effect  of  light  in 


PRINTING  WITH  CHROMIUM         167 

the  reduction  of  the  dichromate,  manganous  salt  is 
oxidized  to  the  manganic  state  and  the  image,  which  is 
visible  and  of  a  brown  color,  is  considered  to  be  com- 
posed essentially  of  manganic  oxide.  This  image  is 
merely  intermediate  and  does  not  form  the  completed 
picture.  The  final  print  is  made  by  putting  a  sheet  of 
carbon  tissue  of  the  desired  color  into  an  acetic  acid 
solution  containing  an  organic  reducing  agent,  mag- 
nesium and  ferrous  salts,  and  bringing  the  face  of  this 
tissue  into  contact  with  the  paper  carrying  the  man- 
ganic oxide  image.  In  the  presence  of  acid  manganic 
oxide  is  reduced,  by  the  ferrous  salt  and  the  developer 
(hydrochinon),  to  the  manganous  condition,  and  the 
gelatine  of  those  portions  of  the  carbon  tissue  which  are 
in  contact  with  the  manganese  compound,  of  which 
the  image  consists,  becomes  insoluble.  The  papers 
are  subsequently  separated  in  water  at  43  degrees 
Centigrade  and  development  carried  out  in  the  ordinary 
way  by  dissolving  the  soluble  parts  of  the  gelatine. 
Since  the  pigment-carrying  gelatine  of  the  carbon  tissue 
is  finally  left  upon  the  surface  of  the  printed  sheet, 
there  is  no  right  and  left  reversal  of  the  image,  and 
this  is  one  of  the  advantages  of  the  process.  Others  are 
found  in  the  visibility  of  the  image  during  printing,  and 
the  brush  control  of  development  which  is  common  to 
the  pigmented  gelatine  methods.  In  1905  Mr.  Manly 
brought  out  his  ozobrome  process,  using  bromide  prints, 
instead  of  light  shining  through  negatives,  for  making 
the  exposure.  By  this  means  the  worker  is  made  inde- 
pendent of  daylight  and  enjoys  the  advantage  of  not 
requiring  enlarged  negatives.  The  image  either  can 
be  formed  upon  the  bromide  enlargement  directly  or  is 
transferable  to  another  support  and  is  non-reversed  in 


1 68  CHEMISTRY  FOR  PHOTOGRAPHERS 

both  cases.  The  second  method  is  generally  preferable 
because  then  the  bromide  can  be  used  over  again  to 
make  other  ozobrome  prints.  If  carefully  handled  also 
it  may  be  entirely  uninjured  in  the  end.  The  peculiar 
feature  of  the  process  is  that  the  pigmented  gelatine 
film  is  so  treated  chemically  that  a  contact  exposure  of 
it  to  the  silver  image  of  a  bromide  print  for  twenty 
minutes  produces  an  insoluble  gelatine-pigment  image 
upon  the  ozobrome  tissue,  which  is  adaptable  to  the 
usual  forms  of  development  with  hot  water. 

In  the  bromoil  process  photography  seems  to  have 
been  carried  to  its  extreme  in  the  way  of  control, 
for  the  pigment  is  applied  with  a  brush.  A  vigorous 
bromide  enlargement  is  prepared,  the  silver  image  is 
oxidized  by  ferricyanide  and  removed  by  solution  in 
thiosulphate  and  washing.  At  the  same  time  the 
portions  of  the  gelatine  film  originally  occupied  by  the 
metallic  image  are  insolubilized  by  reduction  of  di- 
chromate  so  that  there  is  substituted  what  may  be 
called  a  latent  image  in  insoluble  gelatine  which  is 
ink-absorbing.  The  surrounding  gelatine  is  water- 
absorbing  and  therefore  refuses  to  "take  up"  ink. 
The  latter  is  put  upon  the  print  by  gently  dabbing  with 
a  soft  brush,  and  obviously  its  amount  and  its  dis- 
tribution are  wholly  at  the  command  of  the  worker. 

These  last  pigment  processes  are  distinctly  ultra- 
advanced  methods  suited  to  the  worker  of  more  than 
usual  artistic  gifts  and  attainments.  They  are  indeed 
photographically  based,  but  their  chief  dependence  for 
success  is,  after  all,  not  so  much  upon  the  control  of 
delicate  chemical  and  physical  reactions  as  upon  the 
worker's  technical  skill  with  the  brush  and  his  personal 
powers  of  expression. 


CHAPTER  XI 
THE  CHEMICALS  OF  PHOTOGRAPHY 


*\  V  7E  have  employed  the  metric  system  of  measures 
\\  in  this  book  up  to  the  present  point  without  any 
attempt  at  its  justification.  It  does  not,  in  fact,  seem 
likely  that  it  will  need  to  be  justified  in  the  minds  of 
the  majority  of  readers,  even  though  it  be  true  that  most 
photographic  formulas  are  still  given  in  English  units 
for  English  and  American  workers,  and  even  though 
this  majority  may  still  weigh  out  its  photographic 
chemicals  by  grains,  scruples,  and  ounces.  But  if 
justification  be  required  in  the  minds  of  some  and  in  the 
practice  of  many  more,  it  ought  to  be  sufficient  to  say 
that  since  the  use  of  the  metric  system  has  been  found 
to  simplify  things  materially  for  the  scientific  investi- 
gator, it  will  certainly  be  to  the  advantage  of  the 
photographer  to  adopt  it.  On  this  account  all  the 
experiments  and  all  the  formulas  given  in  this  book 
have  been  written  in  metric  units  exclusively,  since 
undoubtedly  these  units  will  be  familiar  enough  to  the 
more  advanced  readers,  and  any  readers  who  have  not 
previously  been  acquainted  with  them  will  quickly 
become  so  in  the  course  of  the  experiments.  Very 
fortunately  it  is  coming  to  be  the  practice  to  express 
formulas  in  both  English  and  metric  units  in  many  of 
the  journals  and  other  publications,  so  that  we  may 
expect  soon  to  find  the  former  measures  left  out  alto- 
gether. Nevertheless,  if  any  one  insists  upon  making 
use  of  his  apothecary  and  avoirdupois  weights,  he  can 
convert  the  metric  amounts  into  the  old  system  by 

i6q 


i;o  CHEMISTRY  FOR  PHOTOGRAPHERS 

use  of  the  conversion  tables  in  the  appendix.  Let  us 
hope,  however,  that  the  simplest  way  out  has  been 
adopted  by  every  reader,  namely,  by  the  purchase  of  a 
set  of  metric  weights  at  the  start. 

Most  photographic  operations  depend  upon  having 
the  necessary  substances  in  solution  in  water,  as,  for 
example,  the  developer  solution,  which  acts  upon  the 
latent  image  by  first  dissolving  it  piecemeal  and  im- 
mediately reducing  and  depositing  the  silver  before  it 
has  an  opportunity  to  get  away;  or  the  intensifying 
mixture,  which  dissolves  out  a  little  of  the  silver  image 
and  puts  in  its  place  some  more  non-actinic  material. 
So  there  are  two  things  at  least  which  it  is  needful  to 
know  about  every  solution,  first  precisely  what  are  its 
components,  and  second  what  are  the  concentrations 
of  these  constituents.  Frequently  also  the  temperature 
of  the  solution  is  important  and  this  is  told  by  the 
thermometer.  There  are  two  ways  in  which  the  con- 
centration of  a  dissolved  substance  can  be  expressed, 
and  in  both  cases  the  matter  is  exceedingly  simple  if 
metric  units  are  employed,  but  not  nearly  so  simple 
with  the  English  units.  As  we  have  shown  in  some  of 
our  experiments,  solutions  may  be  made  up  in  per- 
centages, 10  grams  of  sodium  chloride  dissolved  in 
water  and  diluted  to  100  cubic  centimeters  forming  a 
10  percent  sodium  chloride  solution.  It  is  well  to 
note  that  by  dissolving  10  grams  of  the  solid  in  exactly 
i  oo  cubic  centimeters  of  water  there  is  not  made  a  10 
per  cent  solution  for  the  reason  that  the  resulting 
volume  will  in  such  a  case  be  a  little  different  from 
100  cubic  centimeters.  Taking  a  suitable  volume  of  the 
10  per  cent  solution  and  diluting,  it  is  a  very  easy  matter 
to  prepare  solutions  of  any  desired  percentages  less  than 


CHEMICALS  OF  PHOTOGRAPHY       171 

ten.  The  simplicity  of  this  proposition  comes  from  the 
established  relation  between  the  gram  weight  and  the 
cubic  centimeter  of  water,  one  cubic  centimeter  of  liquid 
water,  at  all  temperatures,  weighing  very  nearly  (al- 
though not  quite  exactly,  except  at  4-  4  degrees  Cen- 
tigrade) one  gram.  There  is  no  such  simple  relation 
found  in  the  English  system.  The  second  method  of 
expressing  the  concentration  of  a  substance  in  solution 
is  based  upon  either  the  molecular  weight  of  the  sub- 
stance or  upon  its  equivalent  weight.  In  this  way 
solutions  are  prepared  which,  volume  for  volume,  are 
chemically  equivalent  to  one  another,  and  this  be- 
comes very  convenient  for  the  chemical  experimenter. 
Thus,  the  molecular  weight  of  sodium  chloride  is  58.46; 
by  dissolving  58.46  grams  of  pure  sodium  chloride  in 
distilled  water  and  diluting  as  exactly  as  possible  to  one 
liter,  we  should  have  a  normal  solution  of  sodium  chlo- 
ride, as  it  is  called.  In  a  similar  way,  i6q.8q  (the  molec- 
ular weight)  grams  of  silver  nitrate  in  one  liter  of  solution 
will  make  a  normal  solution  of  silver  nitrate ;  and  by  mix- 
ing together  one  liter  of  normal  silver  nitrate  with  one 
liter  of  normal  sodium  chloride  there  would  be  formed 
143.34  (the  molecular  weight)  grams  of  silver  chloride. 
So  also  a  thiosulphate  fixing  solution  at  20  per  cent,  con- 
centration is  very  closely  0.8  normal,  since  the  molecular 
weight  of  sodium  thiosulphate  is  248.22,  and  a  normal 
solution  would  contain  that  number  of  grams  per  liter. 
To  illustrate  the  concentration  of  the  substances  pres- 
ent in  a  developer,  we  may  take  a  hydrochinon  formula 
essentially  as  given  by  Schering,  thus : 

Sodium  sulphite,  dry 20 . 5  grams 

Hydrochinon 4.7  grams 

Potassium  carbonate,  dry 65.7  grams 

Water,  to 1000  cubic  centimeters 


i72    CHEMISTRY  FOR  PHOTOGRAPHERS 

A  normal  sodium  sulphite  solution  contains  63.04 
grams  per  liter;  and  normal  potassium  carbonate,  69. 10 
grams  of  the  salt  per  liter.  At  present  it  is  impossible  to 
express  the  concentrations  of  most  developing  agents 
in  terms  of  normal  solutions  because  chemical  research 
has  not  so  far  determined  quantitatively  the  reactions 
between  the  developing  agents  and  the  silver  salt  of 
the  plate.  Until  these  quantities  have  been  measured, 
the  concentrations  can  very  well  be  expressed  in  terms 
of  the  gram-molecular  weights.  Thus  a  solution  of 
hydrochinon  containing  1 10.02,  (the  molecular  weight) 
grams  per  liter  may  be  called  a  "gram-molecular 
solution"  (G.-M.S.).  Therefore  the  Schering  formula 
when  translated  into  molecular  weight  concentrations 
becomes  practically  (N  meaning  normal) : 

Hydrochinon 0.043  G.-M.S. 

Sodium  sulphite 0.32    N. 

Potassium  carbonate 0.95    N. 

Another  developer  frequently  used  is  the  so-called 
Wellington  Pyro-Soda  in  two  solutions : 

NO.   I 

Pyrogallic  acid 100  grams 

Sodium  sulphite,  dry 100  grams 

Citric  acid 8.3  grams 

Water,  to 1000  cubic  centimeters 

NO.  2 

Sodium  carbonate,  dry 40.4  grams 

Sodium  sulphite,  dry 54-5  grams 

Water,  to 1000  cubic  centimeters 

For  use,  4  cubic  centimeters  of  No.  i ,  30  cubic  centi- 
meters of  No.  2,  and  30  cubic  centimeters  of  water  are 
mixed.  When  diluted  for  use  in  developing,  the  solution 
has  the  following  concentrations : 

Pyrogallic  acid 0.05  G.-M.S. 

Sodium  sulphite o .  50  N. 

Citric  acid o.oi  N. 

Sodium  carbonate o .  3  5  N. 


CHEMICALS  OF  PHOTOGRAPHY        173 

Since  normal  solutions  are  all  equivalent  to  one 
another,  and  the  same  is  true  of  o.  i  normal  solutions, 
etc.,  we  can  at  once  compare  the  strengths  of  solutions 
when  they  are  expressed  in  the  fractional  normal  form. 
It  is  seen  at  a  glance  that  the  concentration  of  sodium 
sulphite  in  the  Wellington  formula  is  nearly  double  that 
of  the  other ;  whereas  the  concentration  of  carbonate  is 
only  about  one  third.  The  reason  for  these  differences 
in  concentration  will  be  plain  if  we  recall  the  density- 
giving  properties  of  hydrochinon  and  the  character- 
istics of  pyrogallic  acid,  viz.,  its  propensity  to  dyeing 
the  gelatine,  and  its  oxygen-absorbing  activity  when 
in  alkaline  solution. 

At  first  sight  it  may  seem  that  this  system  of  making 
up  solutions  is  complicated  and  abstruse.  We  shall 
show  in  a  very  few  words  not  only  that  this  is  not  at  all 
the  case,  but  that,  on  the  contrary,  it  is  the  simplest 
and  easiest  method  in  practice,  if  metric  units  be 
adopted.  Let  us  keep  in  mind,  first,  that  a  normal 
solution,  or  a  gram-molecular  solution,  of  any  sub- 
stance signifies  a  solution  containing  a  definite  number 
of  grams  in  a  volume  of  one  liter;  and,  second,  that  this 
definite  number  of  grams  is  always  either  the  whole 
molecular  weight  or  some  fraction  of  the  molecular 
weight  of  the  substance.  So  in  order  to  make  the 
preparation  of  normal  or  fractional  normal  and  gram- 
molecular  solutions  a  simple  matter,  all  that  is  necessary 
to  have  is  a  list  containing  these  definite  weights  per 
liter  for  all  the  substances  we  wish  to  use.  Such  a 
list  we  have  already  started,  and  it  will  be  well  to  tabu- 
late and  extend  it  here.  Further  molecular  weights 
will  be  found  in  the  tables  at  the  end  of  this  chapter. 


174    CHEMISTRY  FOR  PHOTOGRAPHERS 

i.oN.  potassium  carbonate  6g.i  grams  anhydrous  salt 

per  liter 
i.oN.  sodium  carbonate  53.0  grams  anhydrous  salt 

per  liter 
i.oN.  sodium  sulphite  63.0  grams  anhydrous  salt 

per  liter 

.o  N.  potassium  bromide  i  iq.o  grams  per  liter 

.o  N.  potassium  disulphite  55 . 5  grams  per  liter 

.o  G.-M.S.  amidol  IQ7-Q  grams  per  liter 

.oG.-M.S.  duratol  IQO,.I  grams  per  liter 

.o  G.-M.S.  hydrochinon  uo.o  grams  per  liter 

i  .o  G.-M.S.  metol  344-3  grams  per  liter 

i.  o  G.-M.S.  pyrogallic  acid  l2^-°  grams  per  liter 

Assuming  that  3.0  normal  solutions  have  been  pre- 
pared of  sodium  carbonate  and  sodium  sulphite,  by 
dissolving  159  grams  and  189  grams  respectively, 
and  diluting  each  to  i  ,000  cubic  centimeters,  and  that 
we  wish  to  make  up  100  cubic  centimeters  of  pyro- 
soda  developer  to  use  immediately,  we  should  take, 
referring  to  the  fractional  normal  formula  for  this 
developer  given  on  page  171,  12  cubic  centimeters  of 
3.0  normal  sodium  carbonate,  17  cubic  centimeters  of 
3.0  N.  sodium  sulphite,  i  centimeter  of  i.o  N.  citric 
acid  (70  grams  per  liter),  and  add  0.6  gram  of  pyro- 
gallic acid  (100  cubic  centimeters  is  o.  i  of  one  liter, 
and  therefore  a  0.05  G.-M.  solution  will  have  o.  i  x 
0.05  X  126.0  grams,  or  0.6  gram  of  pyro  in  100  cubic 
centimeters  of  solution),  after  which  this  mixture  is 
to  be  diluted  with  water  up  to  a  volume  of  100  cubic 
centimeters.  The  three-times  normal  carbonate  solu- 
tion may  be  made  up  in  quantity,  if  it  is  kept  in  a 
bottle  having  a  tight  stopper  to  prevent  alteration  of 
the  concentration  by  evaporation.  A  rubber  stopper 
is  best  for  this  bottle.  If  it  be  desired  to  make  up 
considerable  amounts  of  three-times  normal  sodium 
sulphite,  the  solution  should  be  put  in  a  number  of 


CHEMICALS  OF  PHOTOGRAPHY       175 

smaller  sized,  full  bottles  with  tight-fitting  stoppers. 
The  concentration  of  sulphite  solutions  will  change 
not  only  by  evaporation,  if  precautions  are  not  taken, 
but  also  from  spontaneous  oxidation  by  the  air.  A 
good  plan  to  follow  is  to  make  up  only  a  half  liter  or  a 
liter  at  a  time,  according  to  the  amount  of  work  that 
is  being  done,  since  sodium  sulphite  keeps  much  better 
in  solid  form  than  it  does  in  solution.  More  of  these 
standard  developer  formulas,  in  the  fractional  normal 
form,  will  be  found  in  the  appendix.  The  advantages 
of  this  system  are  numerous :  it  has  both  simplicity  and 
uniformity;  a  variety  of  different  developers  can  be 
easily  had  on  hand  and  used  without  cumbering  the 
darkroom  and  its  cupboards  with  a  great  array  of 
bottled  developer  solutions;  the  concentrations  can 
be  quickly  varied  by  definite  amounts,  if  one  be  given 
to  experimenting;  and  it  is  economical,  because  there 
are  never  any  prepared  solutions  standing  around  to 
become  oxidized  and  thus  wasted,  if  the  photographer 
takes  a  few  weeks'  vacation  from  his  hobby.  When  a 
particular  formula  has  been  selected  for  permanent 
use,  the  proper  amounts  of  developing  agent  to  make 
the  desired  volume  of  solution  can  be  weighed  out  and 
put  up  in  cork-stoppered  vials.  A  supply  of  these  kept 
on  hand  will  be  found  very  convenient. 

For  containing  solutions  in  bulk  it  is  desirable  to 
have  a  supply  of  0.5  liter,  liter,  and  ^  liter  glass-stop- 
pered bottles,  and  to  use  them  in  the  following  way  for 
making  up  solutions.  Take  one  of  them,  say  a  ^  liter 
bottle,  and  measure  into  it  carefully  with  a  graduate 
i  ,000  cubic  centimeters  of  water.  Stick  upon  the  side 
of  the  bottle  across  the  water-level  a  label  and  mark 
on  this  the  level  of  the  surface,  preferably  in  indelible 


1 76  CHEMISTRY  FOR  PHOTOGRAPHERS 

ink.  Do  the  same  for  the  2,000  cubic  centimeter  level ; 
and  similarly  for  the  other  bottles.  If  2  liters  of  a 
solution  are  required,  dissolve  in  enough  water  con- 
tained in  a  beaker  or  other  convenient  dish  the  weighed 
solid,  filter  this  solution  upon  filter  paper  in  a  funnel 
supported  in  the  neck  of  the  bottle,  pour  a  little  water 
upon  the  filter  after  the  solution  has  run  through  to 
rinse  off  the  paper,  and,  after  this  also  has  run  into  the 
bottle,  make  up  to  the  mark  on  the  label  with  water, 
and  mix  the  liquid  thoroughly  by  shaking  the  stop- 
pered bottle.  Write  upon  a  larger  label  gummed  to 
the  bottle  the  name  of  the  solution,  i.e.,  "3.0  N.  Sodium 
Carbonate,"  for  example.  This  is  a  general  method 
for  preparing  solutions,  to  dissolve  the  solid,  or  solids, 
in  a  portion  of  the  required  water,  then  to  filter,  if, 
necessary,  and  finally  to  dilute  up  to  the  specified 
volume  with  water.  Sometimes  the  solids  have  to  be 
dissolved  in  a  particular  order  to  avoid  precipitation, 
as  is  the  case  with  duratol-hydrochinon  developer. 

It  is  frequently  useful  to  have  at  hand  prepared 
solutions  of  acids  and  alkalies  of  known  approximate 
concentrations.  Subjoined  are  data  for  making  up 
such  solutions. 

1.  Acetic  acid,  concentrated,  99.5%,  17.5  N;  acetic  acid,  dilute, 

5.  N,  71  cc.  of  the  concentrated  acid  diluted  with  water  to 
250  cc. 

2.  Ammonium  hydroxide,  concentrated,  18%,  sp.  gr.  o.qo,  7.3  N; 

ammonium  hydroxide,  dilute,  7%,  sp.  gr.  0.97,  2.  N,  18  cc. 
of  the  concentrated  ammonia  diluted  to  250  cc.  with  water. 

3.  Hydrochloric  acid,  concentrated,  39%,  sp.  gr.  1.20,  12.8  N; 

hydrochloric  acid,  dilute,  20%,  sp.  gr.  i .  10, 6.  N,  46  cc.  of  the 
concentrated  acid  diluted  with  water  to  250  cc.;  hydro- 
chloric acid,  dilute,  10%,  sp.  gr.  1.05,  3.  N,  22  cc.  of  the 
concentrated  acid  made  up  with  water  to  250  cc. 


CHEMICALS  OF  PHOTOGRAPHY        177 

Nitric  acid,  concentrated,  70%,  sp.  gr.  1.41,  16.  N;  nitric  acid, 
dilute,  40%,  sp.  gr.  1.15,  8.  N,  88  cc.  of  the  concentrated 
acid  diluted  with  water  to  250  cc.;  nitric  acid,  dilute,  25%, 
sp.  gr.  1. 1 5,  4.4  N,  50  cc.  of  the  concentrated  acid  made  up 
to  250  cc.  with  water. 

Sulphuric  acid,  concentrated,  0,6%,  sp.  gr.  i  .84,  18.  N;  NEVER 
POUR  WATER  INTO  CONCENTRATED  SULPHURIC 
ACID;  sulphuric  acid,  dilute,  60%,  sp.  gr.  1.50,  p,.  N,  122 
cc.  of  the  concentrated  acid  poured,  a  little  at  a  time, 
slowly  and  carefully,  with  thorough  cooling  of  the  mixture 
during  the  operation,  into  cold  water,  and,  when  cold, 
made  up  to  250  cc.;  sulphuric  acid,  dilute,  33%,  sp.  gr. 
i  .25, 4.  N,  56  cc.  of  the  concentrated  acid  made  up,  as  above, 
to  250  cc.  with  water. 


178    CHEMISTRY  FOR  PHOTOGRAPHERS 

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Appendix 


TABLE  I 

THE  CHEMICAL  ELEMENTS  AND  THEIR  ATOMIC 
WEIGHTS  (1915) 


Elements              At.  Wts. 

Elements             At.  Wts. 

Elements         At.  Wts. 

Aluminum        Al       27.1 

Holmium        Ho    163.5 

Rhodium       Rh    102.9 

Antimony         Sb     120.2 

Hydrogen        H          1.008 

Rubidium     Rb      85.45 

Argon                A        39.88 

Indium            In     114.8 

Ruthenium  Ru    101.7 

Arsenic             As      79.96 

Iodine              I        126.92 

Samarium     Sa     150.4 

Barium             Ba    137.37 

Iridium            Ir      193  .  1 

Scandium     Sc       44  .  1 

Bismuth            Bi     208  .  0 

Iron                 Fe      55  .  84 

Selenium       Se       79  .  2 

Boron                B        11.0 

Krypton          Kr      82.92 

Silicon           Si       28.3 

Bromine            Br      79  .  92 

Lanthanum    La    139.0 

Silver             Ag    107.88 

Cadmium         Cd    112.40 

Lead                Pb    207.10 

Sodium          Na     23  .  00 

Caesium            Cs     132.81 

Lithium           Li         6.94 

Strontium     Sr       87  .  63 

Calcium            Ca      40.17 

Lutesium        Lu    174.0 

Sulphur         S         32.07 

Carbon              C        12.00 

Magnesium     Mg     24.32 

Tantalum      Ta    181.5 

Cerium              Ce    140.25 

Manganese      Mn     54  .  93 

Tellurium      Te    127.5 

Chlorine            Cl       35.46 

Mercury          Hg    200.6 

Terbium        Tb    159.2 

Chromium        Cr      52.0 

Molybdenum  Mo     96  .  0 

Thallium       Tl     204.0 

Cobalt               Co      58.97 

Neodymium    Nd    144.3 

Thorium        Th   232.4 

Columbium      Cb     93  .  5 

Neon                Ne      20  .  2 

Thulium        Tm  168.5 

Copper              Cu     63.57 

Nickel              Ni      58.68 

Tin                 Sn     119.0 

Dysprosium     Dy    162.5 
Erbium             Er     167.7 

Niton               Nt    222.4 
Nitrogen          N        14.01 

Titanium      Ti       48  .  1 
Tungsten      W     184.0 

Europium         Eu    152.0 

Osmium          Os     190.9 

Uranium       U      238.5 

Fluorine            F        19.0 

Oxygen            O        16.00 

Vanadium    V        51.0 

Gadolinium      Gd    157.3 

Palladium       Pd    106.7 

Xenon           Xe    130.2 

Gallium             Ga     69.9 

Phosphorus     P        31.04 

Ytterbium    Yb    172.0 

Germanium      Ge      72.5 

Platinum         Pt     195  .  2 

Yttrium        Yt      89.0 

Glucinum         Gl        9.1 

Potassium      K        39  .  10 

Zinc               Zn      65.37 

Gold                  Au    197.2 

PraseodymiumPr  140.6 

Zirconium     Zr       90  .  6 

Helium             He       3.99 

Radium           Ra   226.4 

TABLE  II 
METRIC  MEASURES  AND  THEIR  EQUIVALENTS 

Standards 

The  metric  standard  unit  of  length  is  the  meter, 
one  ten-millionth  part  of  the  (assumed)  length  of  the 
quadrant  of  a  terrestrial  meridian. 

The  metric  standard  unit  of  weight  is  the  weight  of 
one  liter  (1,000  cubic  centimeters)  of  pure  water  at  its 
greatest  density  (+  4°  C  and  760  millimeters  pressure) 

and  is  called  the  kilogram,  or  kilo. 

185 


1 84    CHEMISTRY  FOR  PHOTOGRAPHERS 


Measures  of  Length 


Milli- 
meters 

Centi- 
meters 

Deci- 
meters 

Meters 

Inches 

Feet 

Yards 

1.0 
10.0 
100.0 
1000.0 

0.1 
1.0 
10.0 
100.0 

0.01 
0.10 
1.00 
10.00 

0.001 
0.010 
0.100 
1.000 

0.03937 
0.39371 
3.93708 
39.37079 

0.00328 
0.03281 
0.32809 
3.28091 

0.00109 
0.01094 
0.10936 
1.09363 

1  inch  mm     2.5399  cm.                1  yard    -  0.9144  m. 
1  foot  -     3.0479dm.               1  mile     -  1  .  6093  km.  (Kilometer) 

Measures  of  Volume 


Cubic 
centimeters 

Liters,  or 
cubic  deci- 
meters 

Cubic 
inches 

Cubic 
feet 

Drams 

Ounces 

Pints 

1.0 
10.0 
100.0 
1000.0 

0.001 
0.010 
0.100 
1.000 

0.06103 
0.61027 
6.10271 
61.02705 

0.000035 
0.000353 
0.003532 
0.035317 

0.270 
2.71 
27.05 
270.46 

0.034 
0.338 
3.381 
33.81 

0.002 
0.021 
0.211 
2.113 

1  dram  =  3.697  cc. 
1  ounce  =  29.572  cc. 
1  pint  -  473.152  cc. 


1  cu.  in.   =-     16.3862  cc. 
leu.  ft.   =»    28.31531. 
leu.  yd. «  764.51341. 


Measures  of  Weight 


Milli- 
grams 

Cg. 

Dg. 

Grams 

Kilo- 
grams 

Grains 

Troy 
Ounces 

Avoird. 
Ounces 

Avoird. 
Pounds 

0.00000 
0.00002 
0.00022 
0.00220 
0.02205 
0.22046 
2.2046 

1.0 
10.0 
100.0 
1000.0 

0.1 
1.0 
10.0 
100.0 

0.01 
0.10 
1.00 
10.00 

0.001 
0.010 
0.100 
1.000 
10.000 
100.000 
1000.000 

0.000001 

o.ooooto 

0.000100 
0.001000 
0.010000 
0.100000 
1.000000 

0.0154 
0.1543 
1.5432 
15.4323 
154.3235 
1543.2349 
15432.3488 

0.000032 
0.000322 
0.003215 
0.032151 
0.32151 
3.2151 
32.151 

0.000035 
0.000353 
0.003527 
0.035274 
0.35274 
3.5274 
35.274 

1  grain  -  0.06479895  g. 
1  troy  oz.  «  31.103496  g. 
1  av.  oz.  -  28.34954  g. 


1  avoir.  Ib.  =       0.45359265  kg. 
1  avoir.  Ib.  -  453.59265        g. 
1  cwt.  -     50.80237689  kg. 


M  illimeter  •»  mm. 
Centimeter  =»  cm. 
Decimeter  -»  dm. 
Meter  ••  m. 


Abbreviations 

Cu.  centim.  =« 
Liter  = 

Milligram      =•= 
Centigram    - 


cc. 
1. 

mg. 
eg. 


Decigram      -      dg. 
Gram  =        g. 

Kilogram       —      kg. 


APPENDIX 


1 86    CHEMISTRY  FOR  PHOTOGRAPHERS 


APPENDIX 


187 


1 88  CHEMISTRY  FOR  PHOTOGRAPHERS 


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APPENDIX 


I8Q 


iqo    CHEMISTRY  FOR  PHOTOGRAPHERS 

TABLE  V 
FIXING  BATH  FORMULAS 

Sodium  thiosulphate,  crystallized,  Na2S2O3.5H2O; 
molecular  weight,  248.21;  percentage  of  Na2S2O3  in  the 
crystallized  salt,  63.709%. 

For  reduction  purposes,  where  the  thiosulphate  is 
oxidized,  as  by  iodine,  to  tetrathionate,  a  i  .o  N  solution 
contains  248.22  grams  of  the  crystallized  salt  per  liter 
of  solution. 

For  the  reaction  of  metathesis  whereby  soluble  silver- 
sodium  thiosulphate  is  formed,  a  i  .o  N  solution  con- 
tains 248.22  grams  of  the  crystallized  salt  per  liter  of 
solution. 

1 .  An  average  formula  for  plates ;  also  for  lanternslides 
Make  an  approximately  0.8  N  thiosulphate  solution: 

Sodium  thiosulphate 200 .    grams 

Water,  to 1000 .    cc. 

2.  For  films;  also  for  plates 

Make  an  approximately  i  .o  N  thiosulphate  solution : 

Sodium  thiosulphate   250.    grams 

Water,  to 1000.    cc 

Leave  plates  and  films  in  the  solution  until  all  creami- 
ness  has  disappeared,  and  then  for  as  long  again.  The 
whole  time  will  ordinarily  be  about  20  minutes. 

3.  For  bromide  and  gaslight  papers 
A  0.8  N  thiosulphate  solution: 

Sodium  thiosulphate 200 .    grams 

Water,  to 1000 .    cc. 

Prints  will  be  fixed  in  10  to  15  minutes. 

4.  For  printing-out  papers 

A  0.4  N  thiosulphate  solution: 

Sodium  thiosulphate 100.    grams 

Water,  to 1000.    cc. 

Soak  prints  for  at  least  i  o  minutes. 

N.B. — These  plain  thiosulphate  solutions  are  to  be  used  at  a 
temperature  of  about  +  1 8°C  (65°F) ;  and  must  be  made  up  fresh 


APPENDIX  iqi 

FIXING  BATH  FORMULAS—  (continued) 

5.  Acid  fixing-bath,   for  plates,   films,  and  developing-out 

papers. 
An  0.8  N  thiosulphate  solution,  containing  potassium  di- 

sulphite  (metabisulphite) : 

Sodium  thiosulphate 200.    grams 

Potassium  disulphite 25 .    grams 

Water,  to 1000 .    cc. 

6.  For  fixing  the  bleached  print  in  the  bromoil  process : 
Approximately  0.4  N  thiosulphate: 

Sodium  thiosulphate 48 .    grams 

Sodium  sulphite 24 .    grams 

Water,  to 500 .    cc. 

Leave  the  print  in  this  bath  2  or  3  minutes. 

7.  Hardening  bath,  using  formaldehyde. 

Formaldehyde 50 .    cc. 

Water,  to 500 .    cc. 

Give  5  minutes;  or  use  y£  the  above  concentration 
and  soak  plates  or  films  for  1 5  minutes. 

8.  Hardening  bath,  with  chrome  alum. 

Potassium  chromium  sulphate 15.    grams 

Water,  to 500 .    cc. 

Immerse  for  10  to  15  minutes. 

N.B. — Ordinary  alum,  potassium  aluminum  sulphate,  should 
not  be  used  for  hardening  purposes,  since  the  results  are  not  so 
permanent  as  with  formaldehyde  or  chrome  alum. 

It  is  much  better  practice  to  keep  the  hardening  solution  sepa- 
rate from  the  acid  fixing  bath. 

Q.     Acid  fixing  and  hardening  bath,  for  negatives  and  develop- 
ing-out papers. 

Sodium  thiosulphate  (o.q  N) 220 .    grams 

Potassium  disulphite .  . 28 .    grams 

Potassium  chromium  sulphate 28 .    grams 

Water,  to 1000 .    cc. 

10     Cramer's  acid  fixing  and  hardening  bath. 

A.  Potassium  chromium  sulphate i  J .    grams 

Potassium  disulphite 22 . 5  grams 

Water,  to 250.    cc. 

B.  Sodium  thiosulphate  (i.o  N) 250.    grams 

Water,  to 1000 .    cc. 

When  solution  and  filtration  have  been  effected,  pour 
A  into  B  with  constant  stirring. 


igi    CHEMISTRY  FOR  PHOTOGRAPHERS 

FIXING  BATH  FORMULAS—  (concluded) 
ii.    To  test  a  fixing  bath  for  exhaustion: 
Place  a  drop  of  the  solution  on  white  paper ;  expose 

it  to  light  and  air.     If  it  turns  brown,  the  solution  is 

worked  out. 

TABLE  VI 

FORMULAS  FOR  INTENSIFICATION 

1 .  Wellington  Silver  Intensifier. 

No.  i 

Ammonium  sulphocyanide 40 .  grams 

Sodium  thiosulphate 40 .  grams 

Water,  to 250 .  cc. 

No.  2 

Silver  nitrate 25 .    grams 

Water,  to 250.    cc. 

No.  3 

Sodium  sulphite 2.0  grams 

Pyrogallic  acid i .  o  grams 

Water,  to 10 .    cc. 

No.  4 

Ammonium  hydroxide  (cone.) 10.    cc. 

Water,  to 100.    cc. 

For  use,  add  1 5  cc.  No.  2  to  the  same  volume  of  No.  i 
with  stirring.  Enough  of  No.  i  is  then  to  be  added  just 
to  dissolve  the  precipitated  silver  sulphocyanide,  and  no 
more.  To  this  mixture  add  2  cc.  each  of  No.  3  and  No. 
4  solutions.  Put  the  dry  negative  into  this  bath. 
Intensification  will  take  place  in  from  5  to  20  minutes. 
Greater  rapidity  and  more  density  can  be  secured  by 
the  addition  of  more  of  No.  4. 

2.  Uranium  Intensifier. 

No.  i 

Uranium  nitrate i .    gram 

Water,  to 100.    cc. 

No.  2 

Potassium  ferricyanide i .  gram 

Acetic  acid  (Glacial) 18 .  cc. 

Water,  to 100.  cc. 


APPENDIX  10,3 

FORMULAS  FOR  INTENSIFICATION—  (continued) 

Mix  equal  volumes  of  No.  i  and  No.  2.  After  in- 
tensification has  reached  the  desired  point,  wash  for 
10  minutes  in  running  water. 

3.    Chromium  Intensifier. 

No.  i 
Potassium  dichromate 5.5  grams 


Water,  to 100.    cc. 

No.  2 
'ater,  to 100.    cc. 


Hydrochloric  acid  (cone.) 5 . 5  cc. 

Wai 


Mix  i  volume  of  No.  i,  i  volume  of  No.  2,  and  2 
volumes  of  water,  just  before  using,  as  the  mixture 
does  not  keep  well.  Bleach  the  negative  completely, 
wash  out  the  chromic  acid,  expose  to  daylight,  and 
re-develop  in  a  developing  solution  suited  to  bromide 
paper. 

4.    Mercury  Intensifies. 

Negatives  must  be  well  washed  after  fixing  and  before 
intensification. 

A.  Mercuric  chloride. 

Mercuric  chloride 2 .    grams 

Water,  to 100.    cc. 

B.  Mercuric  bromide 

Mercuric  chloride 2 .    grams 

Potassium  bromide 2 .    grams 

Water,  to ico.    cc. 

Bleach  the  negative  thoroughly  in  either  of  the  above 
solutions;  wash  for  10  to  15  minutes,  and  develop  the 
white  image  in  daylight  either  in  sodium  thiosulphate 
solution  ( i  in  i o),  or  in  amidol  developer.  Wash  and 
dry. 


194    CHEMISTRY  FOR  PHOTOGRAPHERS 

FORMULAS  FOR  INTENSIFICATION—  (concluded} 
C.    Mercuric  iodide. 

Negatives  do  not  need  a  careful  washing  before  in- 
tensification by  this  method. 

Mercuric  chloride 2 .  grams 

Potassium  iodide 2 .  grams 

Sodium  sulphite 20 .  grams 

Water,  to 100 .  cc. 

The  negative  is  intensified  directly  in  this  bath,  but 
must  be  subsequently  treated  with  a  developer  to 
prevent  fading. 

TABLE  VII 
FORMULAS  FOR  REDUCTION 

1.  For  uniform  reduction,  as  of  over-developed  negatives. 
Permanganate  Reducer 

Potassium  permanganate 0.5  gram 

Sulphuric  acid  (cone.) v 2 .    cc. 

Water,  to 200 .    cc. 

Dissolve  the  permanganate  in,  say,  half  of  the  water, 
add  the  sulphuric  acid  to  this  solution,  and  dilute  to 
the  volume  specified. 

For  use,  dilute  i  volume  of  the  stock  solution  up  to 
4  volumes  with  water.  When  the  density  has  been 
enough  diminished,  rinse  the  plate  and  put  it  for  5 
minutes  in  an  acid  thiosulphate  fixing  bath.  Wash  well. 

2.  For  reducing  shadow  details,  i.e.,  increasing  contrast,  as  in 
over-exposed  negatives. 

Farmer's  Ferricyanide  Reducer 

No.  i 

Potassium  ferricyanide 5  .    grams 

Water,  to 50 .    cc. 

No.  2 

Sodium  thiosulphate 100 .    grams 

Water,  to loco .    cc. 

Since  ferricyanide  does  not  keep  well  in  solution,  it  is 
to  be  made  up  only  as  needed. 


APPENDIX  195 

FORMULAS  FOR  REDUCTION—  (concluded) 
To  100  cc.  of  No.  2  add  5  to  10  cc.  of  No  i,  immerse 
the  plate,  and  rock  the  tray  until  the  negative  has  been 
sufficiently  reduced.    Wash  well. 

3.  For  reduction  of  highlights,  i.e.,  to  diminish  contrast. 

A.  Ceric  Sulphate  Reducer 

Ceric  sulphate  (crystals) 10.    grams 

Sulphuric  acid  (cone.) 4 .     cc. 

Water,  to 100 .    cc. 

Take  i  volume  of  the  stock  solution  and  dilute  with 
i  volume  of  water.  Rock  the  tray. 

B.  Bennett's  Ammonium  Persulphate  Reducer 

Ammonium  persulphate 1 2 .  grams 

Sodium  sulphite 2 .  grams 

Sulphuric  acid  (cone.) i .  cc. 

Water,  to 100 .  cc. 

Dilute  i  volume  of  the  stock  solution  with  4  to  8 
volumes  of  water.  After  reduction,  rinse  the  plate  and 
put  it  for  6  minutes  in  an  acid  thiosulphate  solution. 
Wash  well. 

4.  For  making  bright  bromide  prints  from  weak  negatives. 

Wellington  Iodide  Reducer 

Potassium  iodide 2.0  grams 

Iodine 0.2  grams 

Water,  to 300.    cc. 

After  the  bromide  print,  which  must  be  somewhat 
over-developed,  has  been  fixed  and  washed,  it  is  put  in 
the  above  bath  and  left  until  sufficiently  lightened. 
Then  put  it  in  fresh  thiosulphate  solution  for  a  few 
minutes  and  wash  it  thoroughly.  In  the  iodide  solution 
the  whites  of  the  print  turn  a  dark  blue  owing  to  the 
formation  of  the  iodide  of  starch  with  the  sizing  of  the 
paper.  This  blue  compound  is  destroyed  again  by  the 
thiosulphate. 


ig6    CHEMISTRY  FOR  PHOTOGRAPHERS 
TABLE  VIII 

FORMULAS  FOR  DYEING  PLATES  FOR  COLOR  COR- 
RECTION 

Prepare  stock  solutions  of  dyes,  by  dissolving  i .  gram 
of  each  in  95%  ethyl  alcohol  and  diluting  with  the 
same  to  i  ,000  cc. 


No.  i .  Acridine  orange  No. 

No.  2.  Erythrosine 

No.  3.  Orthochrome  T 

No.  4.  Pinaverdol 

No.  5.  Pinacyanol 


No.  6.      Pinachrome 

No.  7.     Homocol 
No.  8.     Dicyanine 


For  blue-green  and  green 
For  greenish-yellow  and  yellow 
For  blue-green,  green,  and  yellow 
For  green,  yellow,  and  orange 
For  red-sensitiveness;  to  the  ex- 
treme visible  red.    Gives  very 
fast  plates 
Sensitizes  to  line  B.    Plates  are 

faster  than  with  No.  3 
For  red-sensitiveness 
For  infra-red 


Use  Distilled  Water 


Blue-green 
Green 

Greenish-yellow 
Yellow 

Green,  yellow 
Orange 

Red* 

Panchromatic  f 
Effects 

No.  1  200  cc. 
Dil.  to     1     1. 

No.  2         200  cc. 
NH4OH       10  cc. 

Dil.  to           1     1. 

No.  3       2  cc. 
Dil.  to     1     1. 

No.  6     2  cc. 
Dil.  to    1    1. 

No.  4 
No.  5 

No.  7 
NH4OH 

Alcohol 
Dil.  to 

11  cc. 
14  cc. 

11  cc. 
33  cc. 

400  cc. 
1     1. 

No.  4       2  cc. 
Dil.  to     1     1. 

Infra-red  J 

No.  8     2  cc. 
Dil.        1     1. 

Red* 

No.  7     2  cc. 
Dil.  to    1     1. 

*Sensitize  without  light. 

tUse  plates  that  are  not  too  fast;  sensitize  without  light. 
JMake  up  just  before  using,  since   the  sensitizing  power  of   the  solution 
quickly  diminishes. 

When  sensitizing,  put  the  plates  in  a  grooved  trough 
or  tank  containing  the  dye.  A  tray  can  be  used,  if  it  is 
rocked  constantly.  Bathe  the  plates  for  3  minutes; 
wash  in  running  water  for  3  minutes;  and  put  to  dry 
in  a  light-proof  drying  cupboard,  or  in  a  tight  box  with 
fused  calcium  chloride  as  a  dehydrating  agent. 


APPENDIX  ig; 

TABLE  IX 
LIST  OF  APPARATUS  FOR  EXPERIMENTS 

i  Alcohol  lamp,  or  if  available  a  Bunsen's  burner  with  rubber 

connecting  tube 

i  Balance,  capacity  100.  grams  to  0.05  gram 

3  Beakers,  glass,  lipped,  100.  cc.  capacity 

3  Beakers,  glass,  lipped,  250.  cc.  capacity 

3  Beakers,  glass,  lipped,  500.  cc.  capacity 

i  Cylinder,  graduated,  10.  cc.  capacity 

i  Cylinder,  graduated,  50.  cc.  capacity 

1  Cylinder,  graduated,  250.  cc.  capacity 

2  Dishes,  porcelain  evaporating,  No.  o,  7.5  cm.  diameter 

1  Filter  paper,  package  of  100  sheets,  18.5  cm.  diameter 

2  Flasks,  glass,  conical,  125.  cc.  capacity 
i     Funnel,  glass,  10.  cm.  diameter 

i  Litmus  paper,  tube  of  100  strips,  red 

i  Litmus  paper,  tube  of  100  strips,  blue 

i  Mortar  and  pestle,  porcelain,  15.  cm.  diameter 

6  Stirring-rods,  glass,  15.  cm.  length 

12  Test  tubes,  glass,  15.  cm.  x  2.  cm. 

i  Weights,  set,  in  block,  50.  gram  piece  to  0.05  gram 


TABLE  X 
LIST  OF  CHEMICALS  FOR  EXPERIMENTS 

N.B. — No  amounts  are  suggested  for  those  chemicals  which 
are  commonly  found  on  the  photographer's  shelves. 

Quantity 

Acetic  acid,  glacial,  (Sp.  Gr.  1.058),  CP 

Ammonium  hydroxide,  cone.,  (Sp.  Gr.  o.qo),  CP. . . 

Citric  acid,  CP 50.  grams 

Ethyl  alcohol,  95% 

Ferric  ammonium  citrate,  CP 50. 

Ferric  oxalate,  CP..' 50. 

Ferrous  sulphate,  cryst.,  CP 50. 

Gelatine,  photographic 500. 

Hydrochloric  acid,  cone.,  (Sp.  Gr.  1.20),  CP 


i g8  CHEMISTRY  FOR  PHOTOGRAPHERS 

LIST  OF  CHEMICALS  FOR  EXPERIMENTS  (concluded} 

Magnesium,  powder  (from  box  of  flashlight) 

Mercuric  chloride,  C.P 50 .  grams 

Nitric  acid,  cone.,  (Sp.  Gr.  1.42),  C.P 

Potassium  aluminum  sulphate,  cryst.  C.P 50 . 

Potassium  bromide,  C.P 

Potassium  dichromate,  C.P 50. 

Potassium  ferricyanide,  C.P 50 . 

Potassium  ferrocyanide,  C.P 50. 

Potassium  iodide,  C.P 25 . 

Potassium  oxalate,  C.P 50. 

Potassium  permanganate,  C.P 50 . 

Pyrogallic  acid 

Silver  nitrate,  C.P 50 . 

Sodium  carbonate,  anhydrous,  C.P 

Sodium  chloride,  C.P 50 . 

Sodium  sulphide,  cryst.,  C.P 50. 

Sodium  sulphite,  dry,  C.P 

Sodium  thiosulphate,  cryst.,  photographic 

Sulphuric  acid,  cone.,  (Sp.  Gr.  1.84),  C.P 

Uranium  nitrate,  cryst.,  C.P 25 . 

TABLE  XI 
BIBLIOGRAPHY 

ABC  of  Artistic  Photography,  Anderson,  $2.50,  '13,  Dodd. 

^American  Photography  Exposure  Tables,  2?c,  American  Photog- 
raphy 

Book  of  Photography,  Hasluck,  $3.00,  '07,  McKay. 

Bromide  Printing,  and  Enlarging,  Tennant,  25c,  '12,  Tennant  and 
Ward. 

Carbon  Print,  Buchanan,  joe,  *io,  J.  B.  Buchanan,  Los  Angeles. 

Chemicals  in  Photography,  Cowling,  25C,  Tennant  and  Ward. 

Chemistry  for  Photographers,  Townsend,  5oc,  '06,  Tennant  and 
Ward. 

Developers  and  Development,  Tennant,  250,  '12,  Tennant  and 
Ward. 

Elementary  Chemistry  of  Photographic  Chemicals,  joe,  '03. 
Tennant  and  Ward. 

First  Book  of  Photography,  Claudy,  750/1 2,  Nast  and  Co. 


APPENDIX  i  QQ 

BIBLIOGRAPHY     (concluded) 

How  to  Make  Good  Prints,  100/14,  Am.  Photographic  Pub. 
How  to  Make  Oil  and  Bromoil  Prints,  Demachy  and  Hewitt, 

2?c,  Tennant  and  Ward. 

Instruction  in  Photography,  Abney,  $2.50,  '06,  Lippincott. 
Investigations  on   the   Theory  of  the  Photographic   Process, 

Sheppard  and  Mees,  $1.75,  '07,  Longmans. 
Lure  of  the  Camera,  Olcott,  $3.50,  '14,  Houghton. 
Manual  of  Photography,   Fraprie  and  Miller,    loc,    '12,   Am. 

Photographic  Pub. 

Nature  and  the  Camera,  Dugmore,  $1.35,  '02,  Doubleday. 
New  Treatise  on  the  Modern  Methods  of  Carbon  Printing, 

Marton,  $2.50,  '12,  Marton. 
Oil  and  Bromoil  Processes,  Mortimer  and  Coulthurst,  joe,  '08, 

Tennant  and  Ward. 
Photo-aquatint,   or    Gum    Bichromate    Process,   Maskell    and 

Demachy,  5oc,  'or,  Tennant  and  Ward. 
Photographer's  Handbook,  Harrison  and  Douglass,  $1.00,  '08, 

Lane. 

Photographer's  Note  Book,  Lambert,  5oc,  Tennant  and  Ward. 
Photographic  Chemicals  and  How  to  Make  Them,  Taylor,  joe, 

'05,  Tennant  and  Ward. 
Photographing  Flowers  and  Trees,  McFarland,  500,  'i  i,  Tennant 

and  Ward. 

Photography,  Hoppe  and  Others,  $3.50,  'n,  Doubleday. 
Photography  for  Bird  Lovers,  Beetham,  $1.75,  'i  i,  Scribner. 
Photography  for  Students  of  Physics  and  Chemistry,  Derr,  $  i  .40, 

'06,  Macmillan. 
Photography,  Its  Principles  and  Applications,  Watkins,  $2.00, 

*i  i,  Van  Nostrand. 
Pictorial  Landscape  Photography,  Anderson,  $1.50,  '14,  Photo 

Era. 

Platinotype  Process,  Warren,  500,  Tennant  and  Ward. 
Platinum  Printing,  250,  'i  i,  Tennant  and  Ward. 
Saturday  With  My  Camera,  Johnson,  $1.50,  '14,  Lippincott. 
Sunlight  and  Shadow,  Adams,  $2.50,  '12,  Doubleday. 
Unit  Photography,  Steadman,  $2.00,  '14,  Van  Nostrand. 
Wellcome    Photographic    Exposure    Record    and    Diary,    500, 

Burroughs,  Wellcome  and  Co. 


INDEX 


Page 

Acetic  acid 87 

Acetylene 73 

Acids 20 

Adurol 75 

Albumen no 

Alcohol qi 

Alkali 22 

Amidol 75 

Ammonium  chloride 21,  no 

Ammonium  dichromate 140 

Ammonium  hydroxide 19,  39 

Ammonium  persulphate  reducer 106 

Ammonium  sulphocyanide          QQ 

Apothecary  weights 7 

Arrowroot 109,  132 

Auric  chloride 114 

Aurous  chloride 114 

Avoirdupois .       7 

Backed  plates 60 

Balance 7 

Bases .-     .     22 

Bennett's  reducer 105 

Benzene 72 

Bleaches 86 

Blueprint 130,  134 

Bromine 49,  57 

Bromoil 168 

Calcium  carbonate 114 

Calcium  chloride         142 

Calcium  hypochlorite 114 

Carbon 10 

Carbon  dioxide 10,  28,  32 

Carbon  process 157 

Carbon  tissue 158 

Celluloid 45,  59 

Cellulose 109 

Ceric  sulphate  reducer 105 

Chemical  change         1 1 

Chemical  reaction 11,22 

Chlorides 16 

Chlorine 17,  26,  30,  39 

200 


INDEX  201 

Chrome  alum        89 

Chromium  intensifier 101 

Chromium  salts 153 

Citric  acid 87,  151 

Collodion no 

Colloid 38,  153 

Color  sensitiveness,  of  emulsion 52 

Color  sensitiveness,  of  eye 52 

Color  sensitizing 53 

Compound 14 

Concentration 170 

Constancy  of  composition 14 

Constant  density  ratio 71 

Copper  sulphate 98,  125 

Copper  toning 124 

Crystalloid 38 

Cyanin 53 

Definite  proportions 14 

Developer         62 

Developing-out  paper iiq 

Development 61 

Development  of  Carbon 1 59 

Development  of  Gum 164 

Dichromated  gelatine 157 

Duratol 77 

Elements 1 1 

Emulsion 23,  46,  118 

Energy  change 22 

Equation 14 

Equivalents iq 

Ether 159 

Ethyl  alcohol         1 54 

Exposure 54 

Farmer's  reducer         105 

Ferric  ammonium  citrate 125,  130 

Ferric  ammonium  oxalate .      .      .140 

Ferric  chloride 25 

Ferric  ferrocyanide 33 

Ferric  oxalate  .     Y     .     .     .     ..     ......  32,  34,  135 

Ferric  sulphate 33 

Ferrous  chloride    .      .      .      .      „     .      .      .      .      ...     .     25 

Ferrous  ferricyanide 34 

Ferrous  oxalate 32,  35,  135,  136 

Ferrous  sulphate 33,  36,  43,  61 

Fixing  bath 80,  81,  117 

Fixing  process 80 

Fog 65 

Fractional  normal  formulas 172,  173,  174 

Gaslight  paper      .     ".     '.    *.     V   ••*.     v     .     .     .     •     •     .120 


202    CHEMISTRY  FOR  PHOTOGRAPHERS 

Gelatine 37,  46,  50,  153 

Gold  chloride        113 

Glass  plate 45,  59 

Glycerine 145 

Glycerine  development 145 

Gradation        51,  71 

Grain,  of  emulsion 49 

Gum  arabic 38,  153 

Gum-bichromate  process 1 62 

Halation 59,  70 

Hardening  bath 89 

Heat  energy 22,  27 

Hydrazine 58 

Hydriodic  acid 31 

Hydrochloric  acid 19,  39 

Hydrogen 30 

Hydrogen  chloride 20,  3 1 

Hydrogen  dioxide 57 

Hydrogen  iodide 31 

Hydrolysis 114 

Hydrochinon 75 

"Hypo" 81 

"Hypo"  eliminator 90 

Indicator 19 

Intensification 93 

Iodine 31,  50 

Iron  toning      .  127 

Kallitype          147 

Latent  image 109 

Latitude 55 

Litmus  paper         19 

Magnesium 12,  23 

Magnesium  oxide 13,  23 

Mercuric  chloride 94 

Mercurous  chloride 95 

Mercury 96 

Mercury-ammonium  chloride 96 

Meta-compounds 75 

Metathesis       .      . 23 

Methyl  alcohol 140 

Metol 75 

Metric  system 169 

Neutralization 22 

Nitric  acid 33 

Nitrogen  oxide 15 

Normal  solutions 170 

Note  book 6 

Ortho-compounds 75 

Ortol 75 


INDEX  203 

Overexposure 56,  93 

Oxalic  acid 140,  148 

Oxidation ,      4,  24,  32,  57 

Oxidizer 24,  36 

Oxygen 10,  15,  24,  31,  58 

Ozobrome 167 

Ozotype 166 

Paper 45,  IOQ 

Para-compounds 75 

Percentage  composition 14,23 

Percentage  solutions         1 70 

Permanganate  reducer 104 

Phenol 74 

Phosphoric  acid 1 50 

Phosphorus 28 

Photographic  image 5 

Physical  change 10 

Pigment 158,  162 

Pinacyanol 53 

Pinaverdol 53 

Plain  salted  paper no 

Platinic  chloride 138 

Platinotype 134 

Platinum 134 

Platinum  recovery 138,  139 

Platinum  salts 117 

Potassium  alum 121 

Potassium  bromide 41,  4^ 

Potassium  chloroplatinite 136 

Potassium  chromium  sulphate 8q 

Potassium  citrate 125 

Potassium  cyanide 80 

Potassium  dichromate 35,  57,  154 

Potassium  disulphite 85,  87 

Potassium  ferricyanide 33,  125 

Potassium  ferrocyanide 33 

Potassium  iodide        41.46 

Potassium  metabisulphite 85 

Potassium  oxalate 34,  136 

Potassium  permanganate 57.  83 

Precipitate 16 

Preparation  of  solutions 174 

Preservative 86 

Preserving  platinum  paper 14^ 

Prussian  blue         33.  '3* 

Pyrogallic  acid 62,  74,  77 

Ray-filter   . 54 

Reaction 11,22 

Reaction,  photo-chemical 27,  30,  32,  39,  41 


204    CHEMISTRY  FOR  PHOTOGRAPHERS 

Reaction,  reversible 32,  41 

Reducer ,        24,  57 

Reduction 24,  32,  61 

"Reduction"  (of  negatives)         93,  103 

Restrainer 68 

Reversal 56 

Ripening,  of  emulsion 48 

Rochelle  salt 1 50 

Salt 22 

Sawdust  reduction  (of  platinum) 145 

Scheele 39 

Selenium 29 

Selenium  cell 29 

Sensitizer 32,  36,  41 

Sensitizing  (platinotype) 140 

Sepia 121 

Silver 4,  14,  43,  97 

Silver-ammonia  chloride 96 

Silver  bromide 41,43,47,49,57,61 

Silver  chloride 17,39,43,112 

Silver  chromate 103 

Silver  iodide 41,43,47,49 

Silver  nitrate 14,  39,  46,  61 

Silver-sodium  thiosulphate 82,  97,  99 

Silver  sub-bromide 41,49,57 

Silver  sub-chloride 40,  112 

Silver  sub-iodide 4^,  49 

Silver  sulphide 82,121 

Silver  sulphite 64 

Silver  sulphocyanide 99 

Sizing 133,  140 

Sodium  acetate 116,  150 

Sodium  carbonate -63 

Sodium  chloride 15 

Sodium  nitrate 1 8 

Sodium  potassium  tartrate 1 50 

Sodium  sulphide l^^ 

Sodium  sulphite 63 

Sodium  tetrathionate 97 

Sodium  thiosulphate 80,  87 

Spectrum .'•••     -     .     5* 

Starch .     .   109 

Substitution  products »'*-•.     73 

Sulphide  toning 1 2 1 

Sulphur ,  V  .     .    28,  87,  88 

Sulphur  dioxide -i ,'-•-.     .  '  v     .      .     86 

Sulphuric  acid 33 

Sulphurous  acid 86 

Synthesis          *3 


INDEX  205 

Tartaric  acid 86,  1 50 

Toning IIQ,  122 

Toning  platinotype 145 

Turnbull's  blue 34,  131 

Under-exposure 55,  93 

Uranium  intensifier 100 

Uranium  nitrate 35,  125 

Uranium  toning 126 

Uranous  ferrocyanide 35 

Uranyl  ferrocyanide 35,  100 

Vanadium  toning 127 

Washing,  by  decantation 16 

Water,  distilled 16 

Weights 7 


NOV   9   194? 


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